There are lags to fiscal policy. Again, it depends a lot on what it is. If you give a fiscal transfer directly to households, the lag probably won't be very long. If you embark on, say, an infrastructure spending program, the lag is probably going to be a lot longer.
The difference between monetary and fiscal policy is that in monetary policy we really have one target and one instrument. We raise and lower interest rates. We have reasonably good estimates of lags. Yes, there is always uncertainty about those lags. In fiscal policy, there are many different instruments.