Basically there is no problem with your calculation method. But the battery life is heavily related to your application. The packet longth, data rate, output power, duty cycle, and sleep mode factors impact the battery life. I think you can get 1 year life base on these factors' appropriate configuration.
It sounds like you're expecting to have to send a lot of data if you're going to be transmitting for 7.5 s every 15 minutes. If you have a lot of data to send, you can use a short connection interval (7.5 ms) for the highest throughput, you can lengthen it again if you don't have any data to send. You can also use the 2 Mbps PHY to get the speed up.