The later at night the better. I have found them continuing to be rather active immediately on nightfall, but activity has fallen away by about 11pm or midnight, though even then there are a few coming in and out.

My previous house had cavity walls and I occasionally got nests in the wall cavity. I successfully killed off one nest just by blocking their entrance with polyfilla. That was in a wall that rose from a concrete floor, the only nearby window having a masonary sill. Also I think I got that one fairly early. Another nest in a wall rising from a suspended floor and close to a window with a wooden sill, and I think had got to a fair size before I noticed it. When I blocked the hole they started gnawing through the windowsill - noisily - and I could see them down the cracks. And a few started coming up through cracks in the floorboards. I had to reopen the main entrance hole and use poison at night.