I follow the recipe in the first post in this thread: No-fail flax seed gel - I finally did it...TWICE!
On my stove, I only boil the seeds for 2.5 minutes and it comes out perfect. If I boil for any longer, it won't go through my strainer at all. I think my stove might be really, really hot though. The trick for me is to stop boiling once it starts to look like the consistency of egg whites. It hardens a little after it cools down.
I reused the seeds last week. I boiled 1/4 cup of seeds once, extracted the gel, and then put the seeds and another cup of water back in the same pot and did it again to make a double batch. I ended up with more than enough for one week of use. If you want to reuse the seeds on a different day, you have to freeze them to keep them from turning.
I don't put any preservatives in my gel. I about a week's worth at once and store it in the fridge. I only take it out to put it in my hair in the morning.
You can experiment to see whether you like it thick or thin. I think it is a matter of preference.