RS485 frame standard (physical layer)
It is driver output enabled state, like DE pin set HIGH on MAX485. Check RS485 on a Scope.

Common sense dictates that device shall disable transmitter ASAP after last stop bit is sent (to release the bus): check "Auto Switching Link" paragraph on this Maxim tutorial. On the other hand, if line quality is low and this phase is short, the receiving transceiver might not detect start first data bit correctly (stated in the first link).

Besides this, communication protocols over RS-485, for example Modbus requires at least 3.5 character silence between frames (or inter frame gap). So to be on safe side, you might introduce delay of say 0.5-1 char bit (in case of Modbus) after enabling transmitter but before transmitting (and also after end of frame).