In general, I don't expect any problems during the upgrade. But it is always better to have console access at least to both devices when doing upgrade on important cluster. Just to save some time in case something will go wrong. You can read release notes for the versions you are going and see if there are any special upgrade information.
Make sure that you follow upgrade path correctly. You will avoid any config problems.
Save the configuration before you start and during all upgrades, in case you need to do some recover.
Feel free to open proactive case with TAC just for standby so you will have faster response time.
Regarding upgrade time, every device is different, I would say that 5-7 minute per device is reasonable to expect. That's around 15 minutes for cluster per version.