As a ballpark, use them at 500nM (so a 1/200 dilution of your stock).
I usually dilute my primer stocks (at 100uM) 1/10, to create a "working stock" at 10uM, then use that at 1/20 (so 0.5ul per 10ul qPCR reaction).
If you have a pair of primers you know work together well and you only ever intend to use together, the "working stock" can be 10uM of both, which saves a bit of time.