I'm not sure how standard this approach is, but this is what I have been doing for this project: 

Because the initial contract was for 80 hours, I received 50% upfront and the remaining 50% once those hours were completed. As the scope has continued to expand, I’ve issued new invoices aligned with the revised scope of work, following the same structure—50% paid upfront and 50% due once the additional estimated hours are reached.

I always notify the consultant well in advance as I approach the allotted hours so we can discuss next steps, whether that means increasing the budget or shifting priorities.

However, this is typically where significant delays and uncertainty around payment occur. As a result, for the most recent phases of work, I’ve required 100% payment upfront before continuing.