You'll enter this as an asset (Leasehold Improvement) in TurboTax Business just like you would enter a new computer purchase. The IRS sets a period of 15 years for leasehold improvements.
However, if the tenant leaves before the 15 year depreciation period is up, the balance of the un-depreciated leasehold improvement is then deducted in the year the lease is terminated. Therefore, you will not lose any of the deduction if you don't stay in the lease for 15 years.