How fast does Prime work?

Its instant. No worries.

I use it mere seconds before adding the water and haven't had problems with in a few years of using Prime

As above....

Many dose Prime to the tank when they are refilling with the Python. I have never had any problems doing it this way.

Burt

however fast chemical reactions are... it takes that long...

There was a huge thread about this question that had a lot of responses. Some people said its bad for your fish and others said that its no big deal. I guess it probably just comes down to how much chlorine/chloramine your water has in it.

I walked away from the thread feeling that while it could potentially be harmful to add prime to your tank and then add untreated water there was no evidence to suggest that this is actually harmful short term or long term. I feel that if you add the correct amount of prime to the conditions your tap water presents, which is the key in this statement, then tap water straight to the tank shouldn't cause any problems.