REM cycles become progressively shorter (1.5 hrs at first down to 40min) as the hours pass and they certainly don't end at 7 hours as the graph shows
Actually, REM sleep starts at a few minutes and progressively gets longer as sleep does(not well represented in graph). As, you can see in the graph, NREM or Non-REM sleep(stages 1-4) starts at around 90-120 and then gets shorter.
This graph ends at 7 hours because that is what they choose the sleep period to be, certainly everybody doesn't sleep for 7 hours as this is just an example.