I think it depends on the age of the mother.
From other strawberry related posts I think it's about 3-4 years...I'm no expert!
4 years is probably a good average lifespan to maximise productivity - given that they reproduce so freely there seems little point in stretching it beyond that.
Best to start a new strawberry patch if you've been using the same ground for several years (helps to avoid the build up of viruses and other diseases.)
New plants are best left to establish in their first year rather than being allowed to fruit (i.e. remove flowers), so you need to work out a regime to have a mix of new and established plants at any one time.