Solutions for MAX retention time?

Okay, we have the thread about the most difficult solutions to retain. What solutions have you been able to retain for extended periods UNASSISTED and for how long?

(I really haven't found anything I can hold over over 25 minutes in any significant volume with standard equipment.)

Not a solution, but mineral oil enemas (even fairly large volumes) can often be retained for literally hours. As other threads have pointed out, they do tend to be messy.