Considering it's not that hard to have more hit points than your constitution score, thus exceeding it with 100% max. hit points (or even 75%, in fact), such spells would be pretty useless if one couldn't go beyond the -CON limit. However, since these spells merely prevent the character from passing out (thus bypassing the rule that says that a character "passes out due to body shock, once he receives a sum of hits equal to or greater than his concussion hit allotment") and not from
dying (thus
NOT bypassing the rule that says that a character "begins to die, if he receives more concussion hits than the sum of total hit points plus his Constitution stat"), I'd start counting the rounds before death from when the limit hits+CON is reached, not from when the character passes out when reaching -100% hits.
In your example, with 248 HPs and 105 (
105?!?!
) CON, whilst you'd only pass out when reaching -248 HPs, I'd start counting the rounds before death (which are 10 rounds, 10% of 105, according to Ch&CL p9) when you reach -105 HPs.
…meaning you'd not merely "start to die" when the spell ends (as Hurin wrote) or you reach -248 HPs, you may just immediately
die if 10 rounds (or more) had passed since when you reached -105 HPs… and your stats may have started to deteriorate even.