Ilford Rapid Fixer(Liquid)500ml

No.

I don't process a lot of B&W films. I also do it for fun so the result is not very critical. My current opened bottle is more than 5 years old.

This type of information is available from the manufacturer, and more reliable to get it from them than from random people on the Internet. For what it's worth, when I mix a batch of Rapid Fixer, I put a piece of tape on the bottle with the date mixed. When I pull out the chemicals to do a developing session, if my mixed Rapid Fixer is more than about a month old, I throw it out. No idea if that's excessively risky or conservative, but that's what I do. Signed, random person on the Internet.

As per Ilford, 24 rolls of 35mm film in 36 exposures can be processed at a 1+4 dilution

No it does not change color, no fixer changes color, you are thinking about indicator stop bath.

Yes. We suggest a three minute fix in Ilford Rapid Fixer for 1+4 and a five-minute wash for x-ray film usage. But this is something an end-user needs to experiment with.

Full, unopened bottles of ILFORD RAPID FIXER concentrate stored in cool conditions, 5–20ºC (41–68ºF), will keep for two years. Once opened use completely within six months and keep all bottles tightly sealed until used.

Yes