I'm planning on buying this, but will I be able to finish the Main Quest + a few side quests and stuff until Skyrim's release? I also have some games coming out in October, so I'm hoping I will be able to beat the Main Quest without being interrupted. It would be weird to play some of the game then come back a month later.

Certainly, especially if you take advantage of some minimal guidance (occasional posts on this forum and peeks at the Wiki). Be careful, as most of the folks 'rond these parts will warn you... you might find that Skyrim sits on a shelf an extra month or two if you really delve into the lore, game content, mods, etc. of MW.

It depends I think on a couple of things, such as how much time you plan to invest into it. Are you only going to play a couple hours every Saturday, or are you going to be playing a few hours every single day? Obviously that will greatly impact how long Morrowind can hold your attention.

That said, player mods are what really extended the life of Morrowind beyond the normal. I played it on a fairly regular basis from the time it launched almost until Oblivion came out, and it was the player mods that kept it interesting and extended it's life. Plus, I was also taken in by the modding as well, which certainly helped to extend the gameplay even further, as I was now testing my own content. During this whole time I was playing and modding, even MORE stuff was coming out, so I ended up downloading tons of stuff and played it all the way through yet again, and each time it was just as fun and but also different because of what each person contributed.

It also depends on how long you generally keep interest in one particular game. Some people just play through until the novelty wears off and then they move on. Others explore every single nook and cranny and don't consider themselves finished until they have completed literally everything there is to complete. For example, on one play through I used the Morrowind Prophecies strategy guide as a sort of checklist to go through and find every vanilla quest, cave, and other point of interest that was listed. I'm that way in Final Fantasy games too, making sure I find all the secrets, complete all the mini-games, get all the best items, etc. Others are just content to play through the game once, whether they hit everything or not, just to experience the story. So personal preference and play style will probably have the biggest impact on how long Morrowind will last for you.

If you literally want a "quick filler", you'll have the MQ done in under 10 hours easily.

I tend to play whatever game I'm into for a few hours a day. And I am quite the explorer, I like to find everything there is in a game. I guess I'll do the main quest and try to find the cool stuff in the game (Mudcrab merchant for example) Thanks guys!