When balancing a card like this, we have to consider when it would be more useful than other cards with similar effect.
For example, if my deck depends on a small number of very important creatures, I would rather take a few quintessences, rather than Sphinxes, since Quintessence offers better protection against Otyughs, EEs, Parasites/Toadfishes, Maxwells, and a number of other CC.
If I want my whole army to be immortal, I would rather take Anubises, since the Sphinxes are once again vulnerable to repeatable CC.
The only situation when I would prefer Sphinxes is if I want to rush, and use Sphinxes to protect the more vulnerable creatures with better attack/cost ratio, like Silurian Dragons or Deja Vu. In such a deck, I don't care if opponent is able to eliminate all my creatures by first eliminating the Sphinxes, then the remaining creatures, since with my rush deck he won't have time to do so.
So, Sphinxes need to be a valuable addition to a rush deck (a reasonable attack/cost ratio), or else no one will use them. On the other hand, their ability is pretty strong, so the big problem is, once you have a single Sphinx on the field (and thus playing additional Sphinxes does not give you any benefit other than added attack), it would be really bad to have to 'pay' for the ability anyway (through paying a lot for a creature with a low attack).
The best solution I can think of is to make the ability into an activated one, eg. make the Sphinx cost 4
and have an ability
3
(or eg. 2
) - All creatures on your side, except Sphinxes, are immortal for 1 turn.
I'm not sure if we should exclude non-Time creatures from the effect.