how to provide a % amount to delete for Leasehold imporovemnts

In 2003 we spent $250,000 on Leasehold improvemnts (no details of the LH)

In 2006 we spent $260,000 on more leasehold improvement.(walls/build out costs)

In 2011 we spent 57,000 on the leashold improvement(finish upgrade only-carpet, paint wall covering)

Questions:

Is it reasonable to assum that prior year L/H can be delete from the book?

how to determine/provide a % amount to delete the old LH?

thanks,