As far as I know, for Apple products. can purchase Apple Care within 60-90 days or before the standard warranty expires. Apple Care for Mac Air can be brought from EPIC Centre of cos.
For Lenovo, I think they do allow purchase of extended warranty beyond the standard warranty coverage and must do so within the warranty period and subject to Lenovo Singapore approval after inspecting your laptop. That is what I was told by the Lenovo Service Centre customer service guy at Changi. Prices for extended warranty also varies depend on the model of the products. Singapore is ONE of the countries that Lenovo does not allow purchase of extended warranty via internet/online unlike in US and definitely not like APPLE warranty where Apple does not even look or inspect your product for extended warranty but buy at user risk if the warranty claim is unfortunately rejected by APPLE.
That is as far as I understand, not an expert in this but hope that I am giving you the correct info as far as I know.....
Do that note that for Lenovo, buying the extended warranty as a bundle should be much more cheaper than buying it separately I guess.... Pay by credit card if you can and get interest free installments...