First of all, you're being asked something that's above your current knowledge and experience, without considering the implicit difficulty of that request.
There's nothing to be ashamed of, nobody would ask a beginner violin player to perform the full part of Sheherazade in a symphony orchestra, or a pianist to play Scriabin's sonatas a year after they put their fingers on a piano for the first time.
The common assumption is that percussion instruments are "easy". Ask your conductor to play that as they wish (and then explain you how they did it), and see what they actually do.
Spoiler alert: they will almost fail either in doing, or explaining.
Timpani rolls are fundamentally trills. If you've ever carefully listened to a trill by a wind or string instrument, you'll probably have found out that the rhythm of that trill has probably no direct relation with the actual tempo. It usually is "as fast as you can".
There is no real math here: you just play "as many notes as you can, for the duration required".
A professional percussionist is normally able to play sixteenths at ~200bpm (four notes per beat).
This means that if the piece is at 200bpm and they have to play a roll lasting a full 4/4 bar, they will approximately play 16 notes. But if the piece is played at 100, they will play about 32 notes. What if the piece is played at 156? Well, something between 16 and 32 notes. Nobody cares.
How fast can you play? It probably doesn't matter that much at your current level and situation. You're probably near the 100-120bpm range (again, sixteenths, 4 notes per beat), but, luckily, timpani have enough sustain. Interestingly enough, some "rolls" can be more effective when played slower than the player could eventually do.
Now, one possibility could be to play the nearest "math factor" according to your capabilities. Supposing you're able to do sixteenths at 120, a piece played at 156 would mean that you should try to play eights or triplets at that speed. Start by doing that: assuming you can play triplets at 156, and that roll lasts one bar, then precisely play triplets for 4 beats. You can only play eights? Then do that.
Still, that may sound a bit awkward: as said above, a trill is tempo-independent. Playing triplets or eights is a precise rhythm, so, how can a real roll/trill be achieved?
One thing that is often underestimated by percussion players (and, sadly, teachers) is to consider breathing and singing.
You're clearly a beginner of some level, you've got some proper understanding of rhythm, but you're probably missing the "next step", which is the proper relation between tempo, rhythm and perception of note duration (something quite difficult for somebody playing instruments that have no proper sustain control such as percussion ones).
One possibility you can use is to try to "sing" the note for the whole duration of that roll, and in the meantime try to play "as fast as you can" on the timpani.
- get a metronome; if you can, use one that allows setting accents every n-beats and set it to 4, so that you'll hear the down beat of each 4/4 bar; start it up with a reasonable tempo (60-80bpm);
- once you're feeling the tempo, start singing a note (doesn't matter which, but if it's the same of the timpani it's better; it also doesn't really matter how your pitch is correct right now) at the beginning of a new bar, and for exactly one bar (four beats);
- stop singing as the bar ends (aka, a new bar begins), and wait for another full bar;
- restart from point 2;
- do it for a few times, ensuring that your timing is correct;
- do the same for different tempos, not strictly increasing (just test yourself with random tempos);
Then, take your mallets and repeat, playing the timpani while singing:
- just follow the above procedure, just playing the beats along with the metronome; remember to do all these steps at different tempos;
- do the same while playing eights ("double tempo");
- again with sixteenths;
- again with thirty-seconds, if the tempo allows you;
- if you're able to, try with triplets (three strokes for each beat);
Now, do the same while trying to play as fast as you can (but not too fast), while still ensuring that you end playing when the bar ends.
Finally, do the same with different durations. For example, while still using 4-beat bars, try to make those rolls lasting 2, 3, 5, 6 or even 7 beats.
The last step is to do the same with the rolls noted on your score.
Since you're just hours to your concert, my suggestion is to try to actually do sing those rolls even while playing (nobody will probably hear it nor care about it). Remember that your voice and breathing are probably the most natural ways you have to make music, and you should trust them, even (if not especially) when playing percussion instruments.