Thinner flax seed gel: Boil the water and seeds about 5-8 minutes. The gel will thicken over the heat a little. Gel will hang in thin threads from your stirring utensil (see the video). The gel will strain quickly and easily.
This gel will provide some support for your hair, shine, all the good things flax seed gel can do. This thinner gel is good for thin or fine hair, hair that is easily weighed down or tends to be low porosity.
Thicker flax seed gel: Boil seeds 10-15 minutes. Some people boil them longer than that. The gel with be thick over heat, you'll get thicker strings of gel hanging from your stirring utensil. It will not run readily through a strainer, it will require some pressure. I usually mash with a spoon, some people strain through the cut-off foot of nylon stockings. NOTE: having your hands contact the gel while straining through nylon stockings will contaminate the gel! You'll need to re-boil it to kill bacteria if you want a long shelf-life in the refrigerator.
This gel will provide extra support and thickness, it might be too softening or heavy for thin or fine hair or hair that tends to go limp easily or is low porosity. Possibly great for thick or coarse hair.
Overnight-soak flax gel will tend towards the thicker side upon boiling. The longer the seeds are in water, the more mucilage/polysaccharides (gel!) can be extracted, whether that is an overnight soak, a longer boil, or leaving the seeds in the hot water after boiling.
Whether or not more mucilage/polysaccharides (gel!) is better depends on your hair and your personal preference.
Links to flax gel recipes on this blog:
Basic flax seed gel recipes with ideas for add-ins.
Super-Smooth Flax Curl Cream
Flaxseed Curling Cream (uses commercial strong-hold gel)
Flaxseed/Aloe Gel with Protein (scroll down a bit)