I would put the over/under at 10 years. The university can get far more value out of that land, right in the middle of the campus footprint. That's the part you're missing.
This is not just a "sports fan" / athletic department decision. The university would prefer to keep the academic portion of the campus contiguous, and they could put some great facilities in that footprint.
From the government perspective, this is a construction jobs boost to the local economy and is a good use of tax dollars, compared to a lot of other programs. It would certainly enhance the experience for everyone. The cost of the Houston stadium is very encouraging.
And from a fan's perspective, as unique as the Dome is, it could be a much better experience. Imagine how many people we might draw to games if you could actually park and comfortably get to the venue. As our fan base ages, this is not insignificant. We could draw an extra 5,000 easy if it were simply easier to get to football games, and the experience was first rate, with good seating, quality food and drink, wide concourses, luxury boxes, better bathroom facilities.
I think the right size is 45,000 for football (which is about what the seating would be in the current Dome if it were simply seats instead of benches). Have to have 27-30,000 for basketball. One person above posted about movable seating (better than our current bleacher set up, but similar - but with a press box and better camera support facilities, at the top, etc.), so we could create a more intimate basketball experience when we have the really big crowds.