That depends more on your protein (+ how pure it is) and what you intend to do with it (e.g. use higher concentration for storage than for purification and less to nothing for activity measurement).

As I understand, you want it for the determination of your enzyme's activity - in that case 0-10 % is usually used. It depends, whether it would interfere with your readings. For example I had a very sensitive method in the past, where I had to leave out everything except the buffer and salt. And it also depends on combination of the stability of your enzyme + how long you want to incubate. The longer you incubate the more enzyme will be degraded. That's why you add the glycerol, to prevent that. If it is crude homogenite (no purification), it shall be rather stable and you could omit glycerol altogether.

You may try to consult some literature to see what is usually used. Or you may try several reactions, say with 0, 5, 10, 15 % glycerol and compare. That would be the best.