First: How do you know if your phone is turned on if you have no screen? In any case, just unplug the battery.
2. Are you using the same glass again and again or a new one each times?
Because loca is really messy and pratically impossible to clean at 100,I suggest you use a new one to be sure.
To minimize your bubbles you have to gently place your glass on your lcd. And by gently I mean the slowest you can. A spudger or dental pick my help as lever.
If the glass drop too quickly it will create a movement in the glue which will add air. Air in liquid = bubble.
Once your glass is place over your lcd, you can press in the center up to the edge to remove the excess of loca and at the same time you'll remove the bubble.
3. Between 15 to 30 minutes. I dont think you can over cure it. So more is better than less.
My final advice is : dont do loca. It's to unpredictable and i had a few of my screen that eventually stop working.