Your estimated time will fluctuate based on how many people are staking and how many coins are being staked. This varies over time. This really should not piss you off. Again, I learned a lot about staking by reading the thread. You can easily buy clams at the various exchanges (poloniex, bittrex, etc.)
I usually calculate like this: If you take the network weight and devide that by 1440 you get the numer of coins required to stake once a day, on avarage. So if the nework weight is 1.000.000 you need 1.000.000/1440 = 694.44 => 695 clam to stake once a day. Off course this a chance calculation so it could happen you stake several times on a day and nothing a few days in a row.
The network weight is dependend on the total amount of clam used for staking, so that can fluctuate on a daily basis.