Lots of guys sustain their pace for that long. Maybe not the super express bowlers who come off a long run eg. Akhtar and Tait, but I've seen Brett Lee bowl 7 overs quite regularly in his time..., and James Anderson certainly should be able to. And yeah it's not a law or anything, but by crickey if my fast bowler couldn't bowl a 7 over spell without either falling over from fatigue or doing himself an injury I'd be really questioning his fitness (and consequently his place in the team). As a captain you'd hate having a guy like that. What if his first 5 overs were great and he's really causing some trouble? But no, he's not fit enough or too frail to manage 7 overs. You've just thrown away a good spell and it might cost you a lot of runs if your replacement bowler can't replicate the pressure. 7 overs is reasonable because your typical 2 hour session has around 28-30 overs and you generally play 4 bowlers which means your spell will be around 7 overs. Some guys like having 2 spells a session sure, but most will only get one.