I guess it depends on how you define 'relationship'. My answer would be not at all in a temporary affair (even those lasting up to several months or a few years). However, in a marriage or other long-term relationship, a large age difference -- which may not seem a big deal at the start -- I imagine will eventually begin to tell in most (if not all) cases.
For example, a 25-year-old woman who falls for a 55-year-old silver fox will probably start feeling differently about her more mature hubby when she's in her sexual prime at 40 and he's 70. She may well still love him dearly but it may not be the same passionate, sexually fuelled love as when they first met. Of course, if he is in effect her sugar daddy, she probably won't care and will be satisfied by a long line of lovers -- but is that a relationship most people would want? (Yes, there are some high-profile examples where wealthy old men, including one or two notorious Australians, are quite content to have much younger wives who almost certainly fool around with men nearer their own age, but I suggest these are exceptions.)
Reverse the sexes and I would be amazed if a 25-year-old stud who enjoyed servicing a 55-year-old cougar would continue to be monogamous with her even 5 years later, let alone after 15 years when she's 70 (unless he's got a granny fetish perhaps).
Those in an 'open relationship', especially where a lot of money is involved (as above), may well prosper no matter the age gap if they have other things in common and share a unique bond.