"We can look at the adoption of the last major Bitcoin core release to guess how long it might take people to upgrade. 0.11.0 was released on 12 July, 2015. Twenty eight days later, about 38% of full nodes were running that release. Three months later, about 50% of the network was running that release, and six months later about 66% of the network was running some flavor of 0.11."
On what grounds do you think it is reasonable to assume that this update will roll out 6x faster than previous data suggested, as oppose to your own observation of 66% adoption in 6 month. or do you believe 38% node upgrade-coverage ( in 28 days ) on the network for a hard fork is good enough?
There are no harm in choosing a longer grace period but picking one short as 28 days you risk on alienating the nodes who do not upgrade with the aggressive upgrade timeline you proposed.