If every phone sold from today onwards were an ICS phone, and half a million of them were sold every day, it would still take about 14 months for ICS to have a 50% share.
Iirc, Froyo took 8 months, and Gingerbread 12 months (just recently) to reach a 50% penetration of Android devices.
You are forgetting about existing phones upgrading. Admittedly, it can be a long process for the hardware vendors to certify ICS for their phones, but 6 months is probably a reasonable time frame.
One thing that has changed about the relationship between Google and the hardware vendors is that Google now has it as a core objective to get the vendors onto the latest platform. In the past, I don't think that it was a high priority on Google's part.
Iirc, Froyo took 8 months, and Gingerbread 12 months (just recently) to reach a 50% penetration of Android devices.