It depends on whether the work is an improvement that extends the life of the building or is a repair/replacement of existing building components. Any costs for this work would be allocated between your personal residence (not deductible) and the rental units.
An improvement is recorded as a new asset for your rental property and would be depreciated over 27.5 years, the same as the main residence. See Table 1-1 from IRS Pub 527 Repairs and Improvements for examples of improvements.
However, you may be able to write off the improvement costs if you qualify for the Safe Harbor Election for Small Taxpayers. TurboTax will ask you about this election under the Assets/Depreciation section of your Rental Property. If you choose this election, you can expense up to $10,000 in costs that would otherwise be depreciated over a very long time.
A repair/replacement is expensed in the year the work is done.
Once the main residence is fully depreciated, nothing will change except you no longer have depreciation expense to deduct from your rental income. If you sell the property, the depreciation you claimed could factor into the taxable gain on the sale.
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