How long should I let Safe Mode startup run? I keep getting the "disk0s2 i/o error" message and I'm just not sure when to give up and try a different method. I already ran disk utility in Recovery Mode and it said there were no repairs needed.

I would recommend trying to repair the disk from single user mode Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support

Usually that error message is due to a failing hard drive.

You have limited opportunity to attempt to create a backup of your created files.

That is what the SafeBoot or safemode appears to allow you -- at the moment.

Since the hard disk drive exhibits signs of failure or other major issues, plan

on a replacement in the near future. You may be able to get the computer to

start up in a regular full OS X (not safe mode) but consider its hours are limited.

An externally enclosed hard disk drive (with own power supply, not relying on

Mac ports to run it) is a good basic means of which to use a disk utility to make

a copy or a Clone of the current OS X. This may help retrieve an archive that

could be used along with a Time Machine backup, to restore your Mac once you

get a new hard drive installed inside.

Good luck & happy computing! 🙂