Not long. If you want to trap her then use the Kittens as bait, if you can get ahold of a drop trap put them in a carrier or live trap or something else secure and put it toward the back of the drop trap so she has to walk into it to get to them.
It can be pretty intense if the mom is very upset, so steel yourself. Using the kittens to trap her is a great idea. My husband once inadvertently closed the door of a car that had been open for about an hour....long enough for the mom to have put her litter in there while she left them for a while. Her anguish lasted for about 2 days until we figured it out and opened the car. They all survived, thankfully.
How did you get the kittens- in a trap? I would use that same trap as it has their scent in it already. If you don't have a trap, you can always borrow one from animal control. Can you record their cries and then play them in the same area?