I've used Ilford Rapid Fixer for T-Max 400 quite a lot -- at the film strength of 1+4 from concentrate, five minutes is plenty and four is probably okay. Do be aware that t-grain films (T-Max and Delta are the most common) will also use up fixer capacity more rapidly than conventional films, due to iodide buildup in the fixer. If you use primarily T-Max films, you should probably reduce Ilford's stated fixer capacity by at least 1/3, or rely on a clearing test (always with the same T-Max film) -- drop a leader into freshly mixed fixer, and time how long it takes to clear; fix for 3x that time, and test ever couple rolls; when the clearing time is twice what it was originally, it's time for new fixer.
Lately, despite my tight budget, I've found it economical to one-shot (well, correctly, one-session) my film fixer. I've been using Polymax RT, which is a Kodak product intended for automated B&W paper processing; I dilute it to half the strength recommended, use it for one to four films in a single session, and discard it. The $20 jug of concentrate, which contained 5 L, is about 6 months old and about half gone (I use one ounce of concentrate to fix a roll of 35 mm; that means cost to one-shot my fixer is, at worst, about twelve cents). I'll keep using this as long as I can get it locally at a good price...
