Here's a
previous discussion on the topic.
From the looks of things, semi-ablative continues protecting you indefinitely so long as you never take 10 or more basic damage from any single hit, but takes into account
all damage, not just what was stopped by the DR (so if you have Semi-Ablative DR 10 and get hit by a 100 basic damage attack, it's all gone).
Houseruling semi-ablative to be cumulative but only apply against what the DR stopped seems logical and balanced. Otherwise, Semi-Ablative may well be too good at low TL, where 10+ basic damage isn't particularly common. That thread also has a houserule for allowing various levels of ablation (instead of Basic's case of Non- vs Semi- vs Fully-Ablative), which may or may not be of interest to you. At first glance, it seems rather fair and balanced.