Right time to ask for an epidural or gas

Meki
With my first I took the gas for helping me and it worked like a charm for me.. I asked it randomly at 4cm(I was in active labor for I think 3 hours and I was 0cm dilated when my water broke) and they gave me. So I don't really know what is the earliest time you can ask for one. If there is a way you determine please let me know.