I just reserved my entrance time to Saint Marks Basillica and noticed it say you only have 10 minutes? Do they really stick to this? I ask mainly because I chose a time to start with the time the illuminate the mosaics, and I have read that it is from 11:30-12:30 ( I think) and my entrance is from 11:25-11:35. I dont just want 5 minutes of it!

There's a predetermined route that is timed to take 10 minutes. If you're going in peak season (any time before November), it's unlikely that they'll let you dawdle.

If you pay an additional fee, you can go up to the museum and gallery, which is likely to get crowded.

The basilica offers a free tour that lasts about an hour. I think there's one at 11 am. Google >Basilica di San Marco<. Reportedly, if you take the tour, you can skip the line and tell the guard you want the "visita guidata" and he'll let you in.

Well I might just reserve a defferent time then, you cant change it or cancel it

Unless this has changed recently, I wouldn't worry about the time. I was there in last July, and I'm sure I stayed longer than 10 minutes! I did pay for the gallery (worth it, IMO), and the Pala D'Oro, but I never saw anyone checking entry times or "pushing" visitors out.
And you are 100% right to want to go while the mosaics are illuminated--incredible difference.

Just to be safe I reserved it for 10 mins later, only 2 euros. No big deal.

Are those 10 minutes the entire time you have for your visit, or just the time slot for entering?

Entire time.

That's incredible. Haven't been into the Basilica in several years- but we distinctly spent well more than an hour - including the roof, museum etc. Although we were there in early May.