Can I assume that a percentage of the time can be parallelized?
You can never assume anything. 

There are plenty of cases where your work, or a percentage of your work, can be split up into k independent tasks. If you have k processors, then each one can handle one task. If you have k+1 processors, then one will be unused. So it's not just a matter of "percentage of work that can be parallelised", it's also a a matter of "how many parallel tasks can be performed". 

There are plenty of cases where work can be parallelised, but work needs to be done to prepare for this. Let's say it takes time t to prepare for one parallel execution, independent of the amount of work. So with two processors, that preparation takes 2t. With 100 processors, it takes 100t. If the amount of work is fixed, there will be a point where more processors add more preparation time than the savings you get. 

There are plenty of cases where there is a simple sequential solution, and a more complex and slower parallel solution. The total time using one processor and the sequential solution may be x. To use k processors may save y of the sequential time, but add f (k) sequential time for preparation, and use g (k, y) in total parallel time where g (k, y) >> y.