Historically QB's have done better when sitting at least 8 games. But its not a certainly they pan out.
If its close in camp, then play Glennon. Let him start the season, see how he plays. If he's even decent, then he becomes a tradable commodity. People complain about the up front money but that very well could net them a draft pick because his contract becomes very cheap and easy to trade next year.
Unless Trubisky is just blowing him away, then he's likely to sit at least the 1st half of the season.
If you ever read about what happened in Seattle, it was super obvious that Wilson was better. Flynn couldn't grasp the offense and was making big mistakes in practice. He'd run a play, get yelled at for doing it wrong. Wilson would run a play and get attaboys. That happened daily and 100% of the team knew Wilson would start after camp ended. Wilson also understood a lot of the pro concepts for a rookie and benefited playing in NC State and Wisconsin systems.
If that happens then yes, Trubisky would likely start. But Glennon has a lot of experience in pro concepts, including that same offense at NC State as Wilson. He's played in different rp offenses while Trubisky is lacking in that department so I don't believe this is a likely scenario.