There are various factors to keep in mind. However, I would go for multiplying factor of 1.5 to 2.
That is - after 15 years of experience, you should
expect salary somewhere between 22.5 lacs (15 * 1.5) to 30 lacs (15 * 2).
Of course, It also depends how many switches you make, and what is average rise during switch in your city (Bangalore).
e.g. If an employee starts at salary of 5 lac and does not change company. Assuming hike of 10% per year, final package at end of 10 years would be approx 13 lac.
If same employee switches companies every 2 years and getting 30% rise during each switch, then final package at end of 10 years would be approx 18.5 lac (and I'm assuming that within any organization, employee is not getting any rise. If we assume 10% rise per year, then package would be enormous 30 lacs - just at end of 10 years).
In my personal opinion - one should not settle down below factor of 1.5 (i.e. for 15 years of experience, anything below 23 lacs should be unacceptable).
Just my opinion though.
I hope its helps.