There is a great free (gratis and libre) VHDL book called Free-Range VHDL which is quick starter. As a neophyte, I am having difficulties judging the relative rules of thumb when it comes to process blocks.
From the book:
In VHDL, the best approach is to keep your process statements centered arounda single function and have several process statements that communicate with each other. The bad approach is to have one massive process statement that does everything for you. [p. 49]
I'm not sure how long exactly it too long. For instance, while trying to designing a simple 4-bit counter with enable, reset and load value functions, my process statement exceeds 40 lines of code. Since we're targeting actual hardware, we were able to synthesize it without a problem.