I normally do 2+ hours but with some reason behind it.
When I book duos I only do it with one of a few established Indies with whom I am comfortable already and another, new to me, established Indy who is on their list of preferred duo partners.
For this I expect two experienced people who are comfortable together already, and that has so far borne out to be true.
Another reason for longer duos is that I'm older and I take longer between SOGs, so there's intermissions in between.
Then the real risk is that I won't get along with the other SP, but professionalism tends to prevent that and so far, so good.
Since I'm already with an Indy that I know and trust, they tend to curate things should they not go right.
If it were with new-to-me younger agency girls, I'd keep it to 2 hours or less as I feel the risk of having a bad experience increases with that many unknowns.
And yes it is a very expensive hobby indeed!