There are probably more elegant solutions using timer objects, but if the code is reasonably simple (no single function takes too much time, no parfor), then you can get a quick-and-dirty solution using tic and toc:
user_input_time = 3;
count = 0; count_max = 1e9;
tic;
while (toc < user_input_time) && (count < count_max) % always include a failsafe!
count = count + 1;
end
disp(count);
Again, this is only reasonable if the while loop cycles quickly; if you specify 5 seconds but each loop takes 4 seconds, it won't kick out until 8 seconds have passed (i.e. the next time toc is evaluated).
(Incidentally, my computer returns 7558253 after 3 seconds.)
Please accept this answer if it helps, or let me know in the comments if your situation requires a more nuanced timing loop.
