It is up to the developers to decide which subnet their Dapp gets deployed to, but we believe all EVM compatible Dapps should exist on the same EVM subnet, to take advantage of the network effect. Since it is decentralised, teams can build multiple EVM subnets with different requirements/assumptions if needed.
There would be a Ren inter-subnet communication standard, which will be used to communicate between subnets. In saying that, it would be simpler to commuicate with those on a single subnet than between subnets.
The timeline would depend on the final design that gets agreed upon and will be part of the RIP.