Depending on how often you play depends on how often you have to change strings. Elixirs should last 3+ months with heavy playing. I still have the same set on my Takamine that I bought almost a year ago... They need to be changed.

Electric strings aren't made for acoustic guitars. Acoustics are made to use heavier strings because heavier strings create more vibration... which creates more noise... which makes your acoustic guitar sound oh so nice. So make sure you're using acoustic strings. A set of acoustic strings very very very rarely will ever have a gut G string... It's almost always a wound G string. So check and see what kind you're using... I recommend the Elixir Nanowebs... you can get them in a light set... which will be easier to play... but not wuite as loud and not as bassy as larger guage sets.