Usage is computed from the time an instance or resource is active (running) until it is stopped, and you are charged based on the hourly price of the resources you use (GPU/compute, etc.).
If usage is less than an hour, it is prorated based on actual runtime, and the final amount is rounded to standard currency precision (for example, cents).
A dedicated budget alert/spend limit is not available in the billing page right now, but you can control spending by using manual top-ups or setting Auto Top-Up amount and threshold.
If a payment fails, the wallet won’t be topped up, and if your wallet becomes too low, you may not be able to start new resources until you add funds or update your payment method.
You can always see your wallet balance at the top (Balance), and you can view usage-based deductions in Billing & Usage → Usage and billing records in Invoices.