If the character isn't developing, just cooking something they have a recipe for, you might allow them to make 100 doses per test or something. You should impose the penalties for cooking (and, presumably, selling) drugs on them, though - vice cops start to get interested, local gangs and syndicates want to control the character's production, citizens who want the character to stop hire other 'runners to make hits (or just start throwing rocks at your windows until they break, after which the white phosphorus grenade goes in) ....
Going into production for themselves can be a very steep and slippery slope to 'Gee I'm Fuckedsville' unless you're very lenient; considering that cooking can be a way to make a lot of money, this is something you should pay a LOT of attention to, as well as discuss with your players. If they want to play Breaking Bad, that's fine - but the PCs should remember that even right from the beginning Walter White was prized mainly because his 'exclusive' twist on the product was so good; if they could find that out, he'd've been dead for sure.