TBH with the lower stuff I'd rather outgear the everloving hell out of it so I can just crush whatever monster it is in under a minute or less (or your pizza is free). The high rank stuff can be more fun even if they're a bit tankier. Teostra Bow and all its glorious OPness helps with a lot of them as I did the 7* capture steve, 6* Gore, and several other request ones. I'm probably going to try and knock out the two Zinogre quest I have sitting at 7* as well. It kinda sucks that requests behave like urgents where you have to be the one to initiate them to get credit though.
Most of which taking under 20 minutes, but Dreadqueen Rathian II being the outlier at coming in at nearly thirty minutes. That said there's a good bit I could've added to speed it up (like not being cheap and getting some Large Barrel Bombs+).