Dances With Ferrets wrote:
If discarding, how far in advance should the new culture be
started to supply a consistent amount of CO2?

If you really care about the consistence of CO2 production, get yourself
a 2-to-1 gang valve and use two smaller bottles instead of one large
one. Replace the culture in the bottles at different times, so that
their operational periods overlap. That way you won't have any gaps in
CO2 production and the production level will be more even. The downside
is two bottles require more maintenance than one, but not much.

--
Best regards,
Andrey Tarasevich