Basically, you want ro increase it at a slow rate - a millisecond is way too fast as you get 1000 of them each second!
Since your monitor probably runs at 60 or 75Hz, so that's as fast as it can change the image - 60 or 75 times per second.
So set your interval to a maximum of 1/60th or 1/75th of a second: I'd probably go with maybe 50 milliseconds (or 20Hz) and increment the number by a small interval. How small? That's easy to work out:
(final value - start value) / (update frequency in Hz * the number of seconds you want it to take)
So if you want a number to climb from 0 to 100 in 10 seconds, you'd increase it by:
(100 - 0) / (20 * 10)
100 / 200
0.5
If you want integer rises, then you tweak the values to use an integer increment:
(100 - 0) / (x * 10) = 1
100 = x * 10
x = 10
So you'd set the interval for 10Hz or 100 milliseconds.