Depends on local laws and prevailing conditions.

In some places it's legal to ride on either, some places may impose legal restrictions. Even if there aren't legal restrictions you may decide to do something that's technically illegal on the basis you consider it to be safer (personally I've got off the road on a hill where my speed was down to 6mph and traffic was backing up behind me)

If the fastest you can manage is 10mph on a road where traffic would normally be going 40mph then for half a mile I'd take the sidewalk, assuming it wasn't busy with pedestrians. If I could manage 20mph on a road where traffic would normally be going 30mph I'd let the traffic deal with going a bit slower. A few of my local roads have 20mph speed limits and when I'm riding at or slightly above 20mph on those roads I don't even attempt to keep to one side on the basis there's no lawful reason why anyone would want to overtake and letting people squeeze past would put me in danger.

Another factor is the width of the road. I'll look at whether a motor vehicle can safely overtake me within the same lane (and make the associated judgment call of whether I'll have anywhere to escape if they do pass way too close).

Bottom line, there are too many variables to sensibly answer with a one-size-fits-all response.