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How long a power cut can you have without having to worry about the effects?

Alot of variables with that question. I start getting concerned about 10 minutes into it and taking action.

What you should always do if your power goes out is to make sure that the filtration unit does not drain exposing the media inside to air for any length of time causing it to dry out. Also, if your filter has a bio-wheel you don't want that drying out either. Take it off its rocker and toss it in the tank allowing it to just float around.

Depending on the size of tank and your stocking level of fish you may need to substitute the function of your air pump to replenish the oxygen supply to the water. Periodic "blows" with a turkey baster works great for this.

usually, and with "proper" stocking, your fish can live quite a long time without electricity (i'd say a few days). i once got back home from a 2 days trip to find that electricity's out. my tank is small and a bit overcrouded, and planted (meaning plants were consuming oxygen when the lights went off) and i was worried for my fish sake, but nothing happened.
mind you, the fish were stressed (colors faded, fish became quite static), but i think it was the heat that plunged considerablly that got them. they did get back to their old selves once the temp. got over 22C.
as long as the room is warm enough (not going under 15C), and the power cut does not take more than a few hours, you shouldn't worry.