i think airtel are giving speed burst after every few hours that what i have noticed .
yes, ur quite right, on 13th and 14th i got around 240kb/s download speeds for a total of around 12 hours and believe me,i didnt waste even a single minute of those hours, downloaded around 9 GB of stuff, LOL . normally ,i am on Day 512 Kbps/Night 1 Mbps (Turbo 1299) plan.
by the way ,i hope the Turbo 1299 plan also get a speed upgrade by june . i recently got my airtel connection , on 15th march and at that time i hadnt heard of this speed upgrade thing, otherwise i would probably have gone for 512 Kbps plan. it would be really unfair if they dont upgrade the Turbo 1299 plan, as people who have paid less for other plans will be getting better speeds and at cheaper price than those with the turbo 1299 plans. the "minimum' airtel can do with the turbo 1299 plan is make it 1 Mbps unlimited ,otherwise this plan will be useless in coming months after all the speed upgrades. on top of this all, while taking my new airtel connection ,i took the option of paying 6 months advance ,so its not even possible for me to downgrade to 512 Kbps plan .the best all the people with the Turbo 1299 plan can hope for is a speed increase ,hopefully.
so if any person with turbo 1299 plan gets a speed upgrade in future,do let us know.