I normally do a warmup on the bike for 7-10 minutes. This gets my heart rate up to around 120. I then do the elliptical/treadmill for around 30 minutes trying to keep my heart rate above 160 at all times. If you can keep your heart rate up in between each machine I would say go for it. Otherwise if you go 10 minutes then have to wait 3 for the next machine you wont be getting the best aerobic benefit.