This is generally accepted to be about 20 minutes but that's not a hard and fast rule and multiple things can extend that time.
Basically they need to continue writing your citation for speed or a brake light or whatever, at a reasonable speed. If you ever see dash/body cam footage where this is relevant, cops have had charges dismissed because they fill out the entire citation but don't sign it in about 10 minutes, then spend 45 minutes questioning everyone in the car and trying to get probable cause for a search. This is part of why they'll do a lot of the fishing at the beginning before they start the citation process, as that's seen as more "reasonable" than doing it at the end. This is also why it's so important to only give the info you're required to and not to answer any other questions. It increases the odds you'll get that first ticket but it cuts off their ability to extend the stop.
Basically they need to continue writing your citation for speed or a brake light or whatever, at a reasonable speed. If you ever see dash/body cam footage where this is relevant, cops have had charges dismissed because they fill out the entire citation but don't sign it in about 10 minutes, then spend 45 minutes questioning everyone in the car and trying to get probable cause for a search. This is part of why they'll do a lot of the fishing at the beginning before they start the citation process, as that's seen as more "reasonable" than doing it at the end. This is also why it's so important to only give the info you're required to and not to answer any other questions. It increases the odds you'll get that first ticket but it cuts off their ability to extend the stop.