In the context of this forum (adult triathletes, many without a competitive swim background), I 'd say sub 6:00 for men under 55 years old, and sub 6:30 for women under 55.
I think for the majority of triathletes sub 6:00 is a dream that will not ever be attained. More like 6:20 and 6:45/:50 for M/W to be off or near the very front of the AG swim in the majority of races anyone will do. (local/regional/70.3/IM)
To be a FOP M AG swimmer you need to be holding about 1:21-:24 for your longer repeats without falling off in the back end of a 2000+ yard main set without taking a lot of time between reps. If you can do 300,400,500,600 on the 1:25-:27 pace and hold your repeat times you're going to in a very good position in T1.
If you can do repeats on that pace you're going to be out of the water in the top 8 in your AG almost every race you do.
While having a fast 500 or 1000 is great, and will go a long way to getting you to the front of the race, being able to swim within :02/100 for that for 1700-2200yds will put more people behind you after the swim.
Brian Stover USAT L1