Ok, well in order to answer your question we need some information.
What is your recipe?
You say your gravity is 2.5 but that makes no sense. Gravities are measured on a scale that usually goes from 1.0000 (water), to upwards of 1.140 (ton of honey to water ration). Although I don't use Balling/Brix from a quick glance at my triple scale that doesn't seem to be a measurement of that either. Can you explain how you measured gravity and what you used to measure it?
Generally you can be sure your mead is done fermenting (notice I said 'done fermenting' not necessarily done) if you take several gravity readings over a 3-7 day period and they don't change.
If after fermentation is complete you can drink it with a straight face, well then I suppose you can call it done. Just one thing to note though, even if it tastes OK right then and there most if not all meads will get better with age. Most, but certainly not all, meads cant take anywhere from 6 months to 1, 2, 3, 4, or more years to age properly.
I realize your recipe says 3-4 weeks, and that is certainly possible, but without the recipe and your procedure there isn't going to be much we can do for you.
For the question about the alcohol content, that goes back to your gravity readings. If they haven't been done or haven't been done correctly then there is no way to tell without spending a lot of cash for lab tests. That's why if you really want to know the ABV you need to be good about taking readings and taking them accurately.