[Windows Central Question]

and when will MSFT release Windows 10 for older lumia phones

[saurabh7890]

If you want to get by insider then it will be around 1GB+

Regarding official release it is not yet confirmed

[sikurunana]

Of what I've read in the comments, the Preview Build is around 1.2GB, so I'd say the final OTA will be around that size (+/- few hundred megabytes).

Release day of course, even thou everyone is expecting it to be end of Feb, Microsoft did say in first quarter of 2016. So it means it could be anywhere between Jan and April. So I'd say don't have your hopes up. We might have to wait a bit longer that we expect. Also, depending on your country and carrier, the OTA update can delay even after the official release date.