VHDL -- When is a process block too long?
Code is too long if you find that when you need to find a specific part of it (e.g. to make a change or to check exactly what it does) it takes longer than a few moments to locate it.  Like most high level languages, VHDL provides many ways that can be used to split a large block of code into smaller modules; use them whenever you start to find it's getting hard to navigate.