Not for nothing, but you shouldn't be doing any kind of maintenance on a box that has active paying customers on it unless you've scheduled and announced a maintenance window.
Installing new software can break things, and customers tend to become unhappy when things they're paying for break.
In the future you should review the installation documentation (Microsoft has a wealth of this on their website) for potential negative effects like having to reboot, and ideally you would test on a development box before you start touching production hardware.
To answer your question, the only safe thing to do at this point is baby-sit the machine until you reach a window where it can be restarted without annoying your customers. I would not click the "OK" button until you are ready to restart since even if it doesn't trigger an immediate reboot it may inform Windows that a reboot is required (and Windows has a bad habit of popping up those "Your computer will be rebooted in X minutes if you don't tell me to stop" boxes once it's been told it needs to reboot).