Question:
We have XenApp 7.x environment and need to know the recommendations on the scheduled reboot. And why specifically it is required for Citrix servers?

Answer:
The quality and behavior of third-party applications and web services running on a specific server can suggest a demand of more/less frequent restarting of the hosting Windows machine as compared to the machines without these services. Some third-party applications may fail to release the acquired system resources in a graceful and timely fashion. This for example may keep some closed (or crashed) applications, services or drivers (when re-ran), in a pending or starting state which might not be easily fixed without a reboot. 

In practice, Disk I/O, Network and Memory hungry applications when run under high and stressed workload on a system with low resources may render your Windows machine lagging, unstable or crashing. Which may suggest you to restart them sooner. 
If you have to run such faulty applications or have to serve more users than the typical capacity of your hardware/software, or you are forced to co-locate incompatible services into one machine, you may come to such a decision where you have to restart your Windows periodically. In many cases you may adjust the restart period by listening to the user complaints about the server speed! 

So, there is no specific or standard duration or time to reboot a server, however we would recommend you to reboot your server at least once in two or three weeks.
This is not just applicable to only Citrix servers, rather to any Windows machine.

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