Too many, IMHO I've was unable to experiment yesterday-today, and left sgminer working with the 1100/1375 frequencies. After 19h of mining I have: accepteds - 405, HW - 288. So, nearly 50% of work is lost.
If I understanding correctly the process, the miner is working on a share, and if there is a HW error during calculation, all of the work on that share is lost, and the miner should begin from scratch. Taking into account, that calculation of one share takes a few minutes, we are loosing more than half of effectiveness

BTW, Claymore's miner has the parameters "--cl-global-work", "--cl-local-work". What are the corresponding sgminer parameters? I suspect, that --cl-local-work <=> --worksize in sgminer?..