Depends on the unit. The Tobias I use has a regulated supply and the circuit is designed specifically to not need a warm up. One of my units did waver a little through time, but there was a poor connection in the power supply causing it. So, how well your unit behaves depends on its design and how certain you are that it is in 100% working condition.
In practical terms, I check the calibration wedge when I start making readings, then again when I am finished. If they agree, then there probably was no change in response during the intervening time.