Welcome to our forum! Welcome to the addiction lol!
And the lfs
strikes again, if you wait for 4 months to do a water change on your tank all of your fish and plants will be dead. 25% every week should be done, and when the tank is cycling sometimes water changes up to 50% DAILY to keep the fish healthy.
If you can't find a liquid test kit at your lfs
try e bay or maybe some one else here can give you a link to find them on line. You should be testing water daily so when your cycle starts you can start doing water changes to keep ammonia, nitrites, and nitrates in a safe zone.
Also I am not sure what kind of water conditioner you are using but I recommend a brand called Prime. It is multi purpose so you don't have to buy allot of other chemicals. This needs to be added with every water change.
Good luck, don't get cocky with your mom lol but have her read about fish in cycles or allow her to read this thread it may convince her that the sales people at your local fish store don't know how to cycle a tank or care for one as far as that goes. I have found they are there for a pay check and most actually don't know squat about fish or how to care for a tank. Man I have gotten some screwed up advice from the fish store. Thank god I found this forum or my tanks would have never made it.