well i've found the multi-city tool on google flights to be useful for this when i'm planning an ex-EU itinerary because in my experience, as CWS noted, it really varies widely. there are times i've been able to spread out a sector by 30 days with no additional cost, and a majority of the time, anything more than 24hrs usually will drastically increase the price.

but it has been a useful tool fo fudge things around with before the long painful slog of trying to use the BA tool.