When I was getting back in to swimming I couldn't do more than two consecutive lengths of front crawl (I found that slowing down too much screwed my technique) so I would just do a breaststroke length (or lengths) to recover. Over time the number of 'recovery' lengths I needed to do dropped down to zero, and I was able to do 1h+ long swims with no breaks.
When I was getting back in to swimming I couldn't do more than two consecutive lengths of front crawl (I found that slowing down too much screwed my technique) so I would just do a breaststroke length (or lengths) to recover. Over time the number of 'recovery' lengths I needed to do dropped down to zero, and I was able to do 1h+ long swims with no breaks.