How long does it take an unpopped, fluid filled blister from a mosquito bite to heal?

Depends on if pus: If the fluid in the blister is only something like serum, then it should clear out if few days to a weeks. If the fluid is pus, then it needs to be lanced and drained and might need antibiotics. The fluid in this case is associated with redness of skin and pain.

Each person differs: By your age, you must have had experience and that's your best clue. Some folk have an allergic reaction that makes it take much longer. If not, it can be fast - a day or two to be fairly unnoticeable. Total healing can be longer even for them. calamine, dilute vinegar, witch hazel all can help a bit. Benadryl (diphenhydramine) can too.