Two things can happen with leaks in drain lines. One, you don't know how big the leak is, or how big it will get. Over time, it could become a major source or moisture. This could be anything from tap water, to solid waste. Since we don't know where the leak is, or how big it is, it's foolish to make an assumption that it will seal itself over time.
The second thing that can happen to a drain line with a leak is that roots will find it. First they seak out moisture, and from there, they work their way into the pipe itself. Here, where allot of homes were built with clay pipe for drain lines, it's very common. With PVC, it's rare, but it still happens. Especially with thinwall drain pipe. Once that root gets the tinniest bit of itself into your pipe, it starts to grow. This takes years, but once it's started, there's no undoing it. You'll never even know it's happening until you start to notice that the water isn't draining like it used to. If you are lucky, it's outside the house and accessable to dig up. If not.....
While there might not be a leak, from what I've read, I wouldn't go any further until I was 100 percent sure of this. It's too simple and easy to make sure of it right now. There is nothing so pressing moving forward on a house that a day or two wont make a difference. There's going to be allot of other delays anyway, mostly because of weather and conficts between the subs and supplies. It's part of building a house. I'd take the time to do a proper air test and know without a doubt that there isn't a leak of any kind. I wouldn't rely on a guy who said 30 minutes if enough, and I wouldn't rely on anybodies warrentee or guarnatee. Both are meaningless ten years from now when you cant' flush your toilet and the block is under the concrete slab.
Eddie