depends on a couple of factors --
1) are you using the built-in VCP cables, or extended via fiber?
2) is the VC layer-2 only, or performing layer 3 functions as well?
3) what version of code are you running? newer versions have added NSR and NSB which will greatly affect the downtime (should be next to none with these running)
if its layer 2 only then downtime should be very brief -- if STP is in use it will have to recalculate though (unless NSB is being used). if its also functioning as a router, and NSR is not being used, then routing adjacencies will bounce, so convergence really depends on what protocols you are using.
Will