There is no VPN client idle time out "standard". You can't configure one via the Dashboard.
I think you can use RADIUS to pass back an idle timeout. I think I have done this once. If you are using RADIUS authentication, check your RADIUS server if you have this configured.
That that some consumer CPE devices might time out UDP NAT translations after a fixed period of time. So the issue could be because of the home routers in use.
You could consider changing to the AnyConnect VPN client (which has fewer of these issues). Check you are using a compatible MX. Note that AnyConnect licences are something extra you have to buy, but they are not expensive.
https://documentation.meraki.com/MX/AnyConnect_on_the_MX_Appliance