There's really no rules with how often you need to shave your sides, just do it when the length starts to bug you :) but if you want to maintain the length it is, once a fortnight is probably a safe bet. I personally find it easier to get someone else to shave the sides for me, I'm kind of uncoordinated with mirrors and stuff, just hold the long bits of your hair out of the way with hands, clips, whatever so they get a clear line of where the shaved bits stop. A number 4 is probably a good starting length, you can always go shorter from there if you want. Hope this helps! :D