Matty, I change blades when it needs to be changed or after 7 shaves.
Some excellent blades I only get 2 great shaves from, Feather comes to mind, some aren't good for even 1/2 a shave, and others can go for more than 7 shaves. There is no rule. You'll know when you get some experience, in the meantime, get a styptic pencil. Cuts/weepers are just part of learning, and I guarantee that if you keep with it it'll get better and you'll need the pencil less frequently.
When the blade begins "dragging" or "tugging" that's what I look for. Or, if I'm not enjoying the shave. Some blades have an unknown quality and I have no idea what it is... One of the last blades I tested would give me lots of weepers for some unknown reason. OK, my skin is creased from age, but I don't think that was the problem, heck, I love Feathers and 7:00 Yellows. Technique wasn't the problem either. I gave it all sorts of time (shaves) to straighten out, but it wouldn't. I changed the blade to one I like and the problem disappeared. Yup, some blades aren't good for even one shave.
I'm trying to answer your question about when to change blades. It all depends is the best answer I can give.
Another thing to try for the spots might be an alum block. It won't prevent the spots, but it might help after the fact. This isn't a styptic pencil, it's another item that you'll rub on your face after the shave. If you don't want to mail order it you can probably find it in the deodorant section. It's other name is "crystal" deodorant. It's priced all over the board. It's just a chemical in crystal form, so go for price. I find the best bang for the buck is at West Coast Shaving, they have a no brand block for a few bucks, it comes in a clear plastic box. Yup, it can be used as an underarm stench preventer too. One block lasts a VERY long time with even heavy use.