No practical experience obviously, but logically, the amount of discharge is finite, and will flow until it is all gone. The method of collection will not affect the volume of discharge.
It SEEMS a lot less on a tampon as it is absorbed really well and cleverly! With a towel you get to see everything and it can seem like a lot more. But the duration of your period will be the same, usually around 4-6 days.
I think, with a tampon, you tend to carry on using them until you can withdraw a 'clean' one (at least that's what I do).
With a towel, once the main flow has stopped, any traces left inside you will likely as not seep out without your noticing very much. So to that end, you'd probably stop using towels before you stopped using a tampon, and so your period might seem shorter.