Republic, Mesa, Expressjet, and Skywest are in IAH.
Republic: It's the smallest base, with some growth since opening in July, but it's still around half the size of the next smallest base. Recent vacancy awards have needed 6 months of seniority to get it. Reserve has been about 6 months once guys get in. Upgrade here started very senior (>4 years), with a lot of senior transfers from other bases. Half of the seniority list are pre 2008 hires. What that means is, don't expect a line as captain anytime soon. FO side will be a reasonable time to hold, but CA side will depend on some steady growth. Unfortunately the training backlog adds to the wait time. If I were interviewing now, I wouldn't wait several months for a class date at Republic.
Mesa is the only guarantee of IAH at the moment. While they sure do have other issues going on there, choice of base isn't one of them if you live near the airport.
Expressjet does most of the UA regional flying in IAH by the looks of it. But having a lot of senior pilots still there has kept upgrade times high. New airplanes may be what they need to get those upgrades down, but time will tell.
Skywest's IAH base is pretty small, and you'll find rumors of it shrinking on their forum. They do at least post junior pilot dates of hire on the website. Last I checked, it was 6 months to hold it as FO and a 4 year upgrade.