I've got a 5L bottle of Agfa fixer that's been open for over 2 years so far, and probably still about a litre left. I didn't bother decanting the concentrate into smaller bottles or using Protectan. I'd guess the date of manufacture to be at least 5 years ago, because I bought the bottle for 17p when Jessops were having a clearout!

Still works the same as when first opened, and yes I do test the occasional print with selenium toner, and film clearing time just to be sure.