How fast does Lomotil work?

I have a prescription for Lomotil for my IBS given by my primary care physician. The instructions were 1 to 2 tabs, 2 to 3 times a day. I am wondering what kind of schedule to take it on. Right now I am taking it whenever I feels cramps, but it occurred to me that this may not be the best way. I used to take hyoscimine which was a sublingual tablet that dissolved under my tongue. That medication was meant to be taken when I felt cramps and would work in a short period of time. I am wondering if Lomotil is fast acting or slow acting. If it is fast acting then what I am doing right now should be okay, but if it is a slow working medication then I would be better off taking it on a regular schedule. Is Lomotil meant to stop the cramps as they are occurring or is it meant to be a prophylactic drug and work to prevent the cramping?

Thank you