I use Virtual Switches as timers. Also, I would suggest that instead of using a DTH Alexa Virtual Switch, you should use an Edge Virtual Switch if your SmartThings Hub is Edge capable since the DTH is going away. Alexa Routines can be triggered with either Open/Close or Motion capabilities.
As for Edge Virtual Switches, I use @TAustin vEdge Virtual Switches. I like using Motion virtual switches since I don’t use Motion sensors in the STHM, and they don’t appear on the SmartThings app Favorites page status area since they aren’t Open. And, using @TAustin virtual motion devices also has a feature to ‘auto return to inactive’ which is effectivity a timer in itself so I don’t have to set the SmartThings Routine to turn Off the Virtual Switch after X time since the virtual motion device will turn Off by itself.
Yeah, you will need to use a workaround like the one from @DaWeav, I don’t see a stock capability that has those values, and custom capabilities are not recognized by third parties like Alexa.
However, I think a capability to help us with this kind of case would be helpful, so, I’ll open a feature request for it.
Note: Feature requests can take some time to be reviewed and implemented (if they get approved). It’ll depend on the team’s workload.