Does anyone know what time of day that Vodafone tries to renew a prepay plan?

Mother in law is on a $20 prepay plan, so I topup her account the night before, in lieu of it renewing the next day. Unfortunately, if she's already run out of txt/calls, she ends up going into that credit, the renew fails, she stays on casual rates, until a few days later when she tells me her "phone isn't working".

I tend to topup the night before so she doesn't lose any carry over calls/data/txt.