Andropen = test, mast = well, mast
Yea, 20 weeks is no problem. I've gone almost 40 weeks.
Things to monitor:
estrogen
blood pressure
cholesterol
liver function
kidney function
hematocrit / hemoglobin
Of course, when you are on cycle, your cholesterol levels are going to be pretty bad. HDL will be low, LDL will high. It's up to you to decide on how bad you can live with. Cholesterol damage is dose dependent too, so if it's bad at 500mg/week, then it's going to be really bad at 750mg/week. BP will probably be higher as well. If you are on long term, hematocrit may creep up quite a bit, so make sure you stay hydrated. If it's get over 55%, you will need to donate blood. I recommend not drinking very much even on injectables, especially if using for long cycles. If estrogen levels are over 50, you will need to start using an AI like arimidex or letro. Try to keep estrogen in the 20-30 range.
Now, a lot of people will never check any of this. THey just juice year round and hope for the best. Not the safest thing, but lots of people get away with it.