Roughly the length of a subway platform no? Since the VIVA station will be at the South end of the subway platform and the YRT terminal will be at the North end of the platform. I can think of a number of stations where getting on the wrong end of a subway train leaves you with a whole platform length, or near full platform length, to get to an exit, connection, or whatever which happens to be at on end of the station. Or at least requires some walking a planning before boarding. For example, Finch Station from the regional bus terminal to the station, Bloor-Yonge to make the connection the the Bloor line, Spadina to make the connection to Bloor, Kipling to the parking and kiss and ride, Yorkdale to the exits come to mind for me.That seems like a really long walk between the bus terminal and the VIVA stops.
Also I don't think there will be many YRT to VIVA connections, or at least YRT doesn't plan there to be many at this station. I'm sure they expect the majority of the transfers to be YRT/VIVA to subway. I think there will be connections to VIVA nearby anyway.