The short answer
CALC shows a notice in the chat. Your monthly credits refill automatically at the start of the next billing period. With any paid plan, you can buy a credit pack at any time. If you use Free and need more credits before the next refill, you can upgrade to a paid plan. Separately from your balance, a daily usage limit protects your account against unusually high consumption. If you reach it, CALC tells you, and you can continue the next day. On the Free plan, the free allowance also applies per computer and network. If other free accounts on the same computer or in the same office network have already used it this month, CALC tells you so, even if your own balance still shows credits. Paid plans are not affected by this limit.