If I delete photos from my current iPhoto library which have been backed up on time machine, will it be possible to recover these photos from my time machine backup once they've been deleted from my hard disk/iPhoto library? or will time machine eventually delete them next time I backup? does time machine keep my backups for a particular period of time before deleting them to make room for the next round of backups?
By default, Time Machine keeps the following backups:
If there's space constraints then Time Machine will start to delete the backups, but otherwise it keeps them for as long as possible.
To get to the individual photos you will have to have iPhoto as the active window when you activate Time Machine from the menu bar. This lets you go back and see all of the photos. If you did it from the Finder it's more complex because iPhoto bundles its whole library into one big package.
Time machine automatically deletes files off the backup drive in two cases:
The hourly backups fold in to one daily backup - so if a file A was backed up at 2 pm and file B was backed up at 4 pm, even though those backups get deleted, the files all fold into the remaining hourly backup that then becomes that day's backup. Similarly after a month - up to seven daily backups get folded into a weekly backup. You should be able to go back to get any photo you deleted during this period of folding and deleting by going to the closest time to when the file was added or changed.
The real problem is that the system is allowed to delete any backup except the most recent once the backup disk is full. So if you deleted some Fall pictures several months back - if the system feels it needs more space for a backup, it will delete all your old photos. Even if you select the warning, the system warns you after the deletion is complete* that they are already gone. Most of the time, you don't lose the entire history, but I have seen that happen twice
Two friends were really surprised to find one day their time machine only had one hours worth of backup.
One time it was a bug or an error - the system thought it needed 100TB of space and it deleted all the old backups except the one from one hour in the past.
Another was a friend that kept adding external drives over time for photo storage and adding them to Time Machine. The iMac only had 200G used on the 1TB drive, but three external drives with 300G of photos each caused the same deletion of all the old backups. The 1 TB time capsule couldn't hold all the files and deleted all the old backups and then popped up a message that it couldn't even start the backup process.
I don't mean to make it seem like Time Machine is bad - just that I am in the practice of having a second hard drive to make yearly backups of all photos before I do the deleting. Time Machine is great to save me from myself accidentally deleting something as well as protect from a failure of my mac but I don't trust it to be the only storage place for something I may want later.
Time Machine will keep old versions deleted/modified files for as long as it has disk space to do so. There's no fixed period of time, but given that Time Machine only stores files that have changed, the odds are pretty good it will keep it for a long while. It ultimately depends on the size of the your back up drive, the size of the disk you are backing up, and the cumulative size of files you change in the future.