[gsharma]

Not initially. It started with 1 GB at the time the other popular services Yahoo and Hotmail provided 2-4 MB of storage.

Google later changed it to “unlimited” which increased daily by an MB or so. At some point settled at 15 GB.

[htrp]

When did it settle in at 15GB?

[Ekaros]

2013 it seems there is blog about combining 10 GB gmail to 5 GB drive and Google+ Photos...

https://gmail.googleblog.com/2013/05/bringing-it-all-togethe...

Not sure if it was unlimited actually before this point.