Question: when to dump the pipe and clean for new round of smoking

New to smoking (and meth in general) but was wondering when smoking off a simple bulb pipe, when does the average smoker say “let me wash this out and add new product”? I use a torch and am careful not to scorch the thing mainly because I don’t wanna break any glass, so over time the product inside slowly browns, but i’ve been dumping and replacing it when there is still mostly white/clear product - am I wasting it? Ive seen people say just dump it out when it starts to taste bad and get dark brown/black, but what do you guys do? Also, if i add product, smoke a few hits, is it okay to add more product to a used bulb? Thanks for the comments!