Cos there are none!
If you work 1 hour that's counted by SW as if you worked a full day. The reason being you are not available for work on that day.
However at the other end of the scale there are no guidelines, whether you work 6,8 or 10 hours, a day is a day
There are other issues around 3 day week and signing for the other days depending what went before the 3 day week. Were you working full time/signing full time.
It's a complex area so maybe it might worth having a chat SW or Citizens Information