What is the recommended period for reboot of Citrix XenApp servers?

Normally servers only have to be rebooted after updates have been installed that require reboots, and not during normal operation.

If a system requires reboots because otherwise it gets unstable or crashes, then the reasons for that are usually badly written programs that have memory leaks etc. In such a case you should try to find the software that is causing this and replace it with something better, or get the developers to improve it. If currently reboots are required because of such ill behaved software, then that is individually different from environment to environment, and there is no way for us to tell you what the best interval is. You'd have to evaluate it yourself.