Having assisted, I'd say the whole procedure takes 5-10 minutes. However, local (injected) anesthetic takes at least 5 minutes to take effect, so, if it's used and allowed to take effect, the procedure would take about 10-15 minutes total.
It could take longer though too, if the circumcision provider is less experienced (like a resident), if it's being used as a 'teaching case', or if immediate complications (like excessive bleeding) arise.
Jen