Reapers could totally wreck a planet and kill everyone on it in a day or so. It's what they do.
However, assuming that this is more of a siege and extraction of the tech and populace like it seems the Reapers are doing at the beginning of ME3..:
Earth/Humans - A month or two. Most defenses gone within a couple days. Holdouts and ground defenses for up to a month, stragglers for the last month. Shepard has at most a couple weeks in game story time imo.
Thessia/Asari - A couple to a few months, assuming a similar situation as with Earth. Most defenses gone within a week. Holdouts and ground defenses for a month or so, stragglers for a month or so. Things would very quickly unravel here as well, but once asari are on the run, they can be quite formidable for a good while (unlike defenseless human civilians). Biotics will only help for so long though.
Palavan/Turian - Assuming that an evac *doesn't* happen, I give it a few months. Organization and diligence will last longer, but fleets and infrastructure may only give up to another weeks worth of time for people. It will be a fierce and brutal fight, and basically Earth x2 or even x5 in terms of struggle. However, once it looks like things are truly lost, even though they'll fight to the last man... once morale is gone, it's *over*. The asari could flee and go their crazy survival stuff to a point, but the turians NEED their community, structure, and fellowship in some form, or they feel rather lost. (Garrus is an exception, not the rule) A quick fold and bulldozing by Reapers after a couple months.
Sur'Kesh/Salarian - Defenses quickly overrun. Mass casualties. Some good strongholds however, and I'm sure strategic areas could be defended for a while... but otherwise, a quick doom. Stragglers have no chance.
Khar'Shan/Batarian - Assuming that they haven't all died already lol... I'd say roughly the same time as earth, except with a different experience involved. The Reapers may not care though, and just turn the planet to glass.
cont.