Spell durations, when do they start?

[garhkal]

This is mostly for 2e but can easily apply to 1e.

Do YOU, have it that character A casting spell Z, has the BENEFITS of said spell, occur in the round it finishes casting on, AND THUS ITS duration starts then too.
Do you have it's benefits/effects kick in then, but the duration ticker starts in the following full round?
Or something else?

[Halaster Blackcloak]

I have it take effect and begin duration in the same round. So even if the wizard goes last on initiative for that round, that's the first round of the duration.