I have played Morrowind for at least 20 hours and definitely played Oblivion almost 20 hours and tried Skyrim under an hour and never played the other ones.

I screwed around in Arena for about 30 hours. I got the second piece of the staff of chaos and then went to the third dungeon and found out I needed to grind for another 5 hours. Stopped there.

Daggerfall I played about 80 hours. For half of that I never got the Recall spell and suffered through nearly impossible dungeons.

Morrowind I've played a few hundred hours. Same with Oblivion and Skyrim.

Wish I could play Redguard and Battlespire.

In my case:

Arena: I installed and tried it to know how it was

Daggerfall: Never tested it.

Morrowind: I keep playing after 10 years

Oblivion: I played the game once. I could not do it again. I dont like it.

Skyrim: I keep playing since release.

Arena: Never touched it.

Daggerfall: Relatively recently installed it, spent about 200-400 hours so far. Love the Dungeons but don't like the open world.

Morrowind: Thousands of hours starting in 2007, lost count. Favourite game of the series.

Oblivion: Only about 500, my least favourite main game despite being my first.

Skyrim: About 1,000-1,500 hours.

Redguard: Finished this buggy mess with a great story.

Battlespire: 20 mins in it crashed my computer, will I retry it again? no thanks...

Arena -never played it.

Daggerfall - played about 10-20 hours on a friend's system before I had even heard of "Elder Scrolls", got far enough to get interested, and then he deleted the game to make room for something else. I've since tried playing the free download with DOSBOX (several attempts for an hour or so each), but the emulator doesn't like my setup and I can't seem to get it to make attacks.

Morrowind - picked it up on a whim, and have put literally thousands of hours into playing, and probably close to a hundred into modding it.

Oblivion - got fed up with it quickly, and installed a popular overhaul mod just to make it palatable enough to complete the MQ. Probably reached the 75-100-ish hour category.

Skyrim - never played it, and not much interested in the "streamlined" gameplay or the setting. As with glaarg, Steam is a factor, but not the biggest one.

Redguard, Battlespire - never played them.

Only ones I've played are Morrowind, Oblivion, and Skyrim. The rest I haven't tried and likely never will because I don't like gaming on computers.

Arena, battlespire, and the hu ding game I haven't played.
Daggerfall-5 hours into it (mostly in character creation) and haven't gotten out of the first dungeon yet....but I keep getting closer
Oblivion I have beaten the MQ twice and have put in over 300 hours on console and pc.
Skyrim-put in 70 hours unmodded and then tallied up about 400 more with major overhaul mods.
Morrowind-I've put in over 300 hours and those hours are still climbing

Only ones I have played happen to be the triple threat of Morrowind, Oblivion and Skyrim starting with Morrowind. That one game I put so many hours into I lost count; definitely more then a thousand without completing the main quest either. So many character and fun times were had with this game. It is by far my favorite of the series.

Oblivion saw somewhere around sixty plus hours in which I saw myself beat the main quest line and all the various factions across two characters. Definitely my least favorite.

Skyrim is around the eight hundred hour mark now and starting to approach Morrowind territory. As flawed as it is I enjoy it; I have high hopes and expectations for the sixth installment.

I can't tell you how many hours I've spent playing these games, I don't keep track. Listed in order of play I can say;

Daggerfall - it was nearly 4-years of daily game play. Loved the mouse combat controls, weird Ii know. Never really completed the MQ or became overly concerned about leveling, mostly just bummed around doing odd jobs for the various guilds. I recently found an old steno pad journal I kept, tracking towns and dungeons visited from '97 until my DX4-100 DOS machine failed in '01. That included just about every location in the Hammerfell providence and a good many in Wayrest and Sentinel.

Morrowind - spent a year in Vvardenfell when it first came out. Not as intense as Daggerfall. There was a lot missing from Daggerfall that I did not appreciate but still the graphics were a vast improvement and it kept my interest. Enjoyed it until I got a good internet connection, I have a bad habit of reading the end of a book. I looked up and read the ending. Well, I didn't like it, the game was ruined and I shelved it. Early 2013 I urge, created my own race and spear mods then set my avatar loose in Vvardenfell with a mission to discover his lost heritage. Been engrossed in it ever since.

Oblivion - constantly from '07 to 2012. Restarted many times, purely vanilla to heavily modded with each game lasting 250 hours on average. Did complete the MQ once in 90hrs with a heavy armored Bosmer.

Skyrim - less than an hour on my son's PC. Found it bland and uninspiring. That and a couple other reasons leaves me with little desire to play in the land of the Nords.

Arena - started a few times never getting out of the 1st dungeon.

Battlespire & Redguard, non-starters.