The replay files (.RPL) are text files that record things like names, locations and speeds of trucks and objects. Being text it is editable to some degree, most notably the stuff at the very top that includes the name of the track's .sit file, the vehicle count and the names of the drivers and the truck files used.
As for how long of a replay you can save, it's been said that the more memory you have the longer the replay you can save, which makes sense. However, since many things can be recorded in a replay the less you have happening on the track the better your results will be. Having less trucks in action, no trains, and a minimum number of scenery models should allow you to record a longer replay. Having more RAM should help too. Beyond that I have no suggestions.
Oh, lets not forget:
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Too many demo indicies
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Replay Ripper