Time spent concentrating is time spent spending a standard action each round on the concentration, and then making any necessary concentration checks to avoid being disrupted. Obviously, it is best if you have some decent peace and quiet for this, but you could conceivably do it while moving (somewhat slowly), or better, riding (though this may prompt concentration checks, it should be no great difficulty to pass them ad nauseam since taking-101 should be sufficient to succeed). This will obviously become a problem if ambushed.
But really, most Pathfinder characters can spare 8 (extra) hours at some point. Downtime is important to a lot of classes—wizards need time to scribe spells, crafters need time to craft, so on and so forth. Some downtime is expected. In fact, the various demands on characters’ time leads to a sort of natural inclination to divide the day into thirds: 8 hours of adventuring, 8 hours of downtime, 8 hours of sleep.
If you compare the aegis’s re-customization option to another character who actually crafts real armor, 8 hours to redo things is a major bargain—the crafter would often have to start over from scratch if he wanted to change something.
However, if your campaign is different, and downtime isn’t available, then yes, consider houseruling the aegis—and also maybe also the wizard, and creating, and other downtime activities.
- I cannot find any rule one way or the other on taking 10 on concentration checks in Pathfinder; clearly, most of the time it wouldn’t matter since the situations that prompt concentration checks are situations that prevent taking-10 anyway. I do find forum discussion claiming you cannot, but nothing substantive. But since the aegis is a port of D&D 3.5e material where you definitely could take-10 on Concentrate (it was a skill), I would just call that a mistake made in porting and allow taking 10 on this even if you normally can’t.