I played new Vegas, and I'm gonna buy the ultimate edition, but how long do all the Dlcs last all together, I like to explore a lot so take that into account. So could somebody give me a rough estimate of the extra hours I will be getting out of this game

Each DLC is about 3-4 hours of content, on average.

I'd say Dead Money (for an inexperienced player) is 5-6, Honest Hearts is more like 3 (without exploration, plenty of random locations to explore), OWB is maybe up to 6-7, Lonesome Road is about 3-4.

I would say that all the DLCs combined could easily give you an extra 20hrs of play time.
Then if your the explorin' type, all the DLCs can give you a few more hours of much enjoyed time.

I have spent about 5 hours on Dead Money and im only at the tower after you start the Gala.

ALL the hours!

Seriously, all of them have a lot of exploring you can do.

I remember spending around 15h ish doing dead money. I like to explore EVERYTHING.

About 25 hours worth maybe more if you explore more.
I spent nearly 10 hours in Zion park - Honest Hearts - just adventuring and looking for stuff while killing the White Legs for nice supplies.

I'm always amazed at other people's playspeed
Dead Money was 8 or 9 hrs for me, HH around 6, OWB around 12, while LR being the only one where is was in the same range as you, about 4 hrs.