Depends on the process, whether or not they're already expressing, suffer from lactation insufficiency, etc, it varies depending on the treatment also. Since prolactin is a hormone, regular administration is typically good for sustained expression, but can have side effects.
Generally speaking, twice daily administration of r-hPRL will drastically increase prolactin levels and result in expressible lactation fairly swiftly. That said, prolactin on its own has a fairly short half life in the body, meaning it will need to be injected multiple times a day to keep levels sufficient.
However, there's also a newer treatment for prolactin deficiency called Prolactin XL, which is a much longer lasting alternative to regular r-hPRL, possessing a serum half life over 2000 times that of prolactin alone, allowing for much longer sustained lactation periods
Currently there isn't technically any medication or drugs formulated specifically to boost prolactin levels, which is why recombinants were formulated. There are drugs that will aid in raising prolactin levels, but only as a side effect, things like birth control can cause prolactin to rise due to heavily increased levels of estrogen for example. Others like melatonin can have the side effect of raising prolactin hormone levels, but yeah that's a supplement to help you sleep. There's also reglan, but that tends to cause symptoms of severe depression as a side effect. It's a complicated topic, but the easiest and most efficient way right now is for sure recombinants