Microsoft Store officially tells you that your download time has expired after you purchase a Windows license, and wish to re-download / re-install more than 3 months after your purchase.

The official solution is to buy an additional copy of the Windows license which will provide you with a new download URL.

I knows this because I complained to the @microsoftstore twitter account: after purchasing 2 copies of Windows (the crapware-loaded OEM version, the clean one from MS store) I had no intention of purchasing a third.

For some reason they simply couldn't allow me a second download, and sent me a physical disc, which arrived about 6 weeks later, by which time a pirated Windows 7 was working fine.

However

I've never had any problems with the copy of windows 7 I bought from the microsoft online store. I've had to redownload it two or three times now, and it's always been there (although the page is near impossible to find). I had a screenshot of the page, but imgur is down today so I can't link it.

Actually, apparently it's 12 months until you can no longer download the OS again:

"Be kindly informed that the availability of the software download, purchased at the Microsoft Store, is limited to one year.

Please understand that as you placed your order more than one year ago, Microsoft Store does not have access to your order and cannot reset the download link."