This website has the longest URL allowed by the domain name registry

boing
13

I couldn’t easily find any data on a standard accepted max length for a URL, the limits for this can be found in web browsers (I assume most modern ones are no longer limited in any meaningful way) and in web servers.

This page (via the webarchive) from 2006 recommends staying safe by staying within a 2.000 character limit: https://web.archive.org/web/20120419143749/http://www.boutell.com/newfaq/misc/urllength.html

That, old and conservative limit, would produce a URL with this length:
a.b.c.d.e.f.g.h.i.j.k.l.m.n.oo.pp.qqq.rrrr.ssssss.tttttttt.uuuuuuuuuuu.vvvvvvvvvvvvvvv.wwwwwwwwwwwwwwwwwwwwww.xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy.zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz.me/a/b/c/d/e/f/g/h/i/j/k/l/m/n/oo/pp/qqq/rrrr/ssssss/tttttttt/uuuuuuuuuuu/vvvvvvvvvvvvvvv/wwwwwwwwwwwwwwwwwwwwww/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz/me/a/b/c/d/e/f/g/h/i/j/k/l/m/n/oo/pp/qqq/rrrr/ssssss/tttttttt/uuuuuuuuuuu/vvvvvvvvvvvvvvv/wwwwwwwwwwwwwwwwwwwwww/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz/me/a/b/c/d/e/f/g/h/i/j/k/l/m/n/oo/pp/qqq/rrrr/ssssss/tttttttt/uuuuuuuuuuu/vvvvvvvvvvvvvvv/wwwwwwwwwwwwwwwwwwwwww/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz/me/a/b/c/d/e/f/g/h/i/j/k/l/m/n/oo/pp/qqq/rrrr/ssssss/tttttttt/uuuuuuuuuuu/vvvvvvvvvvvvvvv/wwwwwwwwwwwwwwwwwwwwww/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz/me/a/b/c/d/e/f/g/h/i/j/k/l/m/n/oo/pp/qqq/rrrr/ssssss/tttttttt/uuuuuuuuuuu/vvvvvvvvvvvvvvv/wwwwwwwwwwwwwwwwwwwwww/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz/me/a/b/c/d/e/f/g/h/i/j/k/l/m/n/oo/pp/qqq/rrrr/ssssss/tttttttt/uuuuuuuuuuu/vvvvvvvvvvvvvvv/wwwwwwwwwwwwwwwwwwwwww/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz/me/a/b/c/d/e/f/g/h/i/j/k/l/m/n/oo/pp/qqq/rrrr/ssssss/tttttttt/uuuuuuuuuuu/vvvvvvvvvvvvvvv/wwwwwwwwwwwwwwwwwwwwww/xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy/zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz