Measure at the one month mark. It’s all up to you but the key is to record your gains while also giving feedback about your routine. If you measure in a month and have not gained anything, then maybe you should seek advice on your routine to make sure you’re understanding the newbie routine right. One month may be too soon to check too, so if you can hold out, wait six or eight weeks.
The masturbation question gets different answers. My opinion is that it’s OK, but I try not to since it makes overall EQ go down for me. If your young and get erections too easily when you jelq, then letting off some steam at the end of a session might be a good idea.
Oh, and take a good picture with a ruler for proof later - even if it’s just for yourself. And read, read, read everything you can here.