Coming from someone that was basically on for 8 months (yes my dumb ass) you should NOT extend your cycle. I am just recently feeling recovered. Believe me its NOT FUN to have barely any libido for months when you are 22. It would be better to cut your cycle to the original 12 weeks you have planned. Come off and do PCT with nolva/sustain rather than waiting for hcg. By the time you get it, the results you get using it at 24 weeks will be less than not using it at 12 weeks. Get recovered, then think about another cycle.
If I recall correctly this is your first cycle right? If so, you have no idea how well or fast you will recover. You could permanently lower your natty test levels by staying on that long (won't necessarily happen but honestly why risk it?). I honestly have no idea whether I'm actually back to where I was before I cycled because I never got tested before. I can only go on that I feel normal again and checking my levels now to see if they are in "normal" range.
Obviously you can do whatever you want, but you REALLY REALLY REALLY shouldn't go longer than 12 weeks. If you wanna talk about it at all just shoot me a PM.