Further research has determined that the limit is around 250 characters:

https://steemit.com/unanswerablechlorimetersdeafengamesiestepacridunreactivephotoelectrodedenticulationsmezcalinesarrogatedboutiqueyirredentaexcelsiorsrouladesgraffednowisekebelescassouletspinochlesaggrievingteadsslimmerdisulfatessclerodermataleftistsplatbandheliotypyot/@powmonkey/okay-the-limit-for-a-primary-tag-is-around-250-characters-but-that-is-still-way-too-much

That's still completely unacceptable.