A/C is cycling about every 15 seconds, so bought one of those cans with a guage inline and watched the pressure as the compressor cycled on and off. When the compressor was off, the pressure was about 45, but when it turned on it would drop down below 25. I added some coolant and didn't see the pressure change at all, but the compressor did stop cycling and would stay on and it seemed to blow cooler. Both lines going/returning from the evap were now cool and sweating, however it didn't seem like much coolant was used from the can.

What should the pressure read on the guage while the compressor is running?
How long does it take to add a can of coolant? (a had that can on about 10 minutes and it didn't seem to make a dent in it)

Any suggestion appreciated in advance.