I do it either when the information/design on the old one is no longer accurate, or when the art on the old one is outdated in terms of art level, but honestly I'm way too lazy to ever do my own ref sheets xD It's too much work. I never let anyone else do ref sheets for me either, tho, because they wouldn't understand exactly how my character is supposed to look haha.
I commissioned one because the artist was doing them in a different style, usable for animation purposes. I felt it was different enough to stand out, and yet still be useful (I resubmitted it to my gallery on the 2nd)
I would like to say that I do it when the information is old or my art is old however I am such a procrastinator. I always think to myself "Oh I should redo that ref" meanwhile I have a bunch of other characters that don't even have a ref for yet. Making references is just so much work, especially because a lot of my characters have two "forms" I guess. I wish I could commission somebody to make them for me but i'm way too poor and I feel a closer bond to my characters if I make their ref myself, if that makes sense.
I used to only redo references when I changed the character's design. But right now I'm working on giving all my pre-2015 characters new references. So far I'm up to 326 omg. That's pretty good for a year and four months xD I have so many more to do tho ;v;
A problem for me is that I have a bad habit of making way too many changes to my character's designs if I don't like something, especially my main sona, Quazar. I can't make a new ref every time I make one change, or else no reference would last more than a couple months, and many would be gone within a few weeks. I've only had references for my characters for about six months, but since then I have updated them once (Quazar twice) and plan to do so every five to six months, or even more often than that, just because I can never be happy with anyone's design.
I never have other people make my ref sheets, but I make new ones when there are big design changes or when the old one looks ugly as sin to me. Then again, the majority of my OCs don't have refs...or are even posted on the internet XD
I wish I could say "I do it either when the information/design on the old one is no longer accurate, or when the art on the old one is outdated in terms of art level.", but honestly, I really only do it when the design is no longer accurate. Bienie is the only one who gets updated refs because of outdated art, and even then I'm not too consistent in remaking her reference sheet. Many characters of mine don't even have a reference sheet yet, and I'll see if I ever get to make them one.