how often a player sends a letter, takes action, and receives a letter, are all things that need to be figured out!
would you be unhappy with waiting two weeks between turns? what about a month?
International mail for letters is fairly fast, though, isn't it? 4-5 days from Sweden, 6-7 from the UK - I'd assume Europe is pretty much all in that range. Asia is probably worse off, but can't be entirely terrible.
so say ~7 days for a message from server to player. even if player responds immediately that's another ~7 days back.
in this scenario the server is waiting for a response before sending the next message, presumably because the player action will affect the next message.
for messages that aren't affected by player action sure, you could send them out a faster rate.