when you run your defragger, you should end up with very low (if not 0%) fragmentation. this is an acceptable level of fragmentation. you will not run into any issues regarding disk performance for weeks if not longer depending on your usage habits
when you defrag every handful of days, you are spending hundreds, thousands, tens of thousands the amount of time you’re theoretically saving yourself. obviously, if you set your machine to defrag while you’re asleep or whatever, you aren’t subject to this. but you’re still only saving yourself probably a couple dozen microseconds/week
in the year of our lord, 2015, i should not have to tell you that you only defragment spinning hard disk drives and never SSDs. defragmenting is an operation that specifically deals with information that is inefficiently spread out across multiple, noncontiguous physical locations on a disk platter. things like files are almost always read sequentially and entirely at once, meaning your disk would read faster if the head didn’t have to seek all over the disk to get all the chunks of the file. SSDs don’t suffer from this issue, they don’t have a “head” that “seeks” and any block of data anywhere on the drive can be accessed in constant time regardless of the current state of the drive
if you try to defrag an SSD, you will needlessly write to it an enormous number of times. this kills the SSD. SSDs can only withstand a certain number of “writes” before the transistors holding your data crap out and your drive dies. if you use your SSD correctly, i.e. don’t defrag it and make sure your OS is correctly issuing the TRIM sata command, you will never hit this number. at least, not on newer SSDs and as long as you’re not constantly writing to it for whatever reason
hard drives, spinning rust ones, are much more resilient to writes, but no matter the disk’s mechanical format, writes are always the most strenuous and taxing operations. a spinning rust drive that incurs writes constantly will die quicker than a similar one that doesn’t.
but defragging will only negligibly hasten your drive’s demise. i promise you, a myriad of other things will kill it, most probably you making a mistake. if you manually defrag your disk every day, again, you are spending several minutes of your time in order to gain just a few a microseconds, which is stupid. best practice is to have your OS periodically check fragmentation statistics, preferably when you’re asleep, and have it defrag automatically once it passes a certain threshold (maybe around 7%)
the best thing to do in any case would be to buy an SSD though. i have never seen a bigger performance boost in any piece of computer hardware i have ever purchased in my life, and i purchase an obnoxious amount of computer hardware