Design begining phase..

You don't. You start coding and you run timing analysis. If you have paths that don't meet timing, then you look at how much combinatorial logic you're cramming between registers and try to fix it(there are Quartus settings that can help, look at the Tools -> Advisors -> Timing Optimization Advisor to get started). Even if you're always making timing, run timing analysis now and then and look at your critical paths. You'll quickly develop a sense of how much logic your coding and how fast it runs. It won't be exact, but you should develop a pretty good feel. The bottom line is you're going to have to dive in, start coding, and see where it goes.