However long it takes them to return to the old url.

You can speed up the process by building a few quality backlinks to the old URL, If the backlinks are on pages that get reindexed consistently, it shouldn't take long for Google to pick up on the 301 redirect URL.

I did something similar here to fix a site that was down, as soon as I built those 3 backlinks my pages started to rank again because I got the bots attention (links on pages that consistently get reindexed). I only used the 3 new backlinks for getting the bots attention, I'll probably remove those 3 links sometime next week when the site that was down gets settled down in the SERPs.