If you were to complete this game, and all the side quests, what would the length be?
There's no exact number to the length of gameplay. It can vary depending on how much time you spend fighting, collecting resources, and travelling through hyperspace.
In any case, you have until 2159, until the Kohr-Ah starts "cleansing" the other races. For myself, that was enough time to meet the Orz, get the code to kill the probes, start the revolution, get the Ilwrate and Thraddash to kill each other, repair the Ultron, tell Talana who really destroyed their homeworld, get the Utwig bomb, etc.
To do everything (everything being all the side quests) takes about a year and a half of game time, if you know what you're doing. At least that's been my experience. If you take short-cuts, it is possible to do it in about six months.