The length of the mix as far as NRG as Rabbi said tend to be shorter. It really all depends on the structure of the track, and the order you put them in. If you were to mix several tracks by tidy trax you could have fairly long mixes (for hard house anyway). Like Rabbi said also you can tell the breakdowns/buildups by looking at the record. With Tidy Trax for example (mainly cause i have just about every record by them) have only one maybe two builds or breaks (or shiny areas, if you will) on the record. Like i said the length of it will really depend on the records, as far as, how long does your next track go before it's first build. A good way to get a feel for it is after the main build up drops back into the song , wait till the end of the first stanza of 8 and drop the first beat of your next track. Again like Rabbi said, you really can't go wrong with 8's, essp. w/house. From there you can determine where exactlly is the best spot to drop it to make them run together smoothly. You may have to wait 8 more, maybe 16. Just play with it and you'll see. It makes alot more sense when you do it.