Im kinda new to the bigger outboard arena ( a little background on asking the question).

I was at Bass Pro today getting some stabil and pickin up some necessities from the 20% off zoom sale and see a bottle of power trim and steering fluid. It got me wondering, how often should I check or change this and where is the reservoir for it anyways. Its somethin I know very little about. Could someone shine some light on the situation in general for me?
Of note- i have a 99 evinrude ficht 150

Thanks in Advance
Adam