Received notification about this yesterday:

How does Data Stack work?

Data Stack rewards you with 100MB* bonus data when you add any eligible Prepaid Value Pack.

The longer you keep renewing your packs, the more data you’ll stack – up to a whopping 10GB*.

*From 4 April 2025 Data Stack will go up to 200MB every 28 days. The maximum amount you can stack will reduce from 10GB to 3GB. If you’ve already stacked more than 3GB by 4 April 2025, you’ll continue to get the amount you’ve stacked as long as you keep renewing an eligible Prepaid Value Pack. 

There are a few annoyances with spark mobile such as the lack of vowifi overseas and very poor coverage in remote areas. But I've stuck with them and used a second sim when traveling, mainly because I have a large and growing data stack.