I’m not sure if Alexa helper is set up for this or not, but it’s easy to do with a virtual timer.
You just create one virtual switch and use the power allowance feature in smart lighting so that it always turns itself off after X minutes.
Then you set your other real switches to follow that switch using smart lighting.
Then you authorize the virtual timer switch for echo to use.
Then when you want to turn off the lights, tell echo to turn on the timer and when the timer automatically shuts itself off, the other lights will shut themselves off too.
You might have to play around with the names until you find one that feels natural to you. “Alexa, turn on lights out timer” or whatever.
There’s an article in the communitycreated wiki on how to create a virtual timer that should help:
Core can do this much more elegantly, but it can be done without it.