In addition to the DAG, you also need to store all the blocks themselves. Using the
--fast pruning option for
parity v1.7 nightly (built early May), I'm seeing about 10x more writes than reads in terms of MB/s and about 2x more reads than writes, in terms of IOPs. I'm also recording about 100KiB-200KiB of writes per transaction (as opposed to per block). At around the present average activity of 50 transactions per block, that works out to 28GiB of writes per day. If you are concerned about SSD wear, this is probably the big one.
However, the actual size of the blockchain, when pruned, grows far less than 28GiB/day. The pruned Ethereum blockchain occupies about 11GiB on my laptop.
I guess one could infer from this that, assuming no growth in transaction rate and a good wear-levelling algorithm, every multiple of disk size larger than the block chain, you buy an extra 0.5 to 2 year of wear -- a 32GiB disk gives your disk an expected life of 1.5 to 6 years if the blockchain is currently 11GiB. (Using data from http://techreport.com/review/27909/the-ssd-endurance-experiment-theyre-all-dead)