SharePoint read-only after running over storage: what "Confirm purchase" does, what frees space

Disclosure: I work on M365 Scouts, which sells read-only PowerShell audits for Microsoft 365 licensing and storage. Nothing below needs our tools; it is from Microsoft Learn.

Learn says a tenant that keeps operating above its SharePoint storage limit risks read-only mode. The limit is the tenant pool: 1 TB plus 10 GB per eligible licence, plus any Office 365 Extra File Storage, in binary GB. Archived data and sites in the recycle bin do not count toward the active quota.

If you have bought storage and the banner is still there:

Confirm purchase. If it is offered after a completed purchase, it removes read-only mode for up to 14 days while the new capacity is applied. It can be used once in a 365-day period, so size the purchase before you click. Learn gives no purchase-to-quota time for commercial tenants.

Grace period. Learn gives commercial tenants no day count. The Education guidance does (30 days at 100% of the pool). Do not assume you have one in writing.

Admin centre lag. Learn says storage figures exclude the last 24 to 48 hours of changes.

What frees space, in the order I would try it:

1. Empty the site recycle bins. Their space counts toward the total until they are emptied or items age out.
2. Trim version history. It only reaches versions that no retention policy covers.
3. Check Preservation Hold. On sites under retention, deleting does not free space until the retention period ends.
4. Archive cold sites with Microsoft 365 Archive. It is billed at $0.05/GB/month against $0.20 for active storage, and it does not count toward the active quota.

If you move to pay-as-you-go SharePoint storage, the tenant is treated as within quota, so the warning emails and banners stop. Set an Azure budget alert, because those warnings were your early signal. Disconnecting while over quota resumes enforcement immediately.

Question for others: after buying storage, how long did it take before your sites left read-only?