1040's with prior year under expense deprec on one rental

an the average, how long does it take you to prepare the 3115?

About 2 hours.
This sample is a good primer, customize it to the actual corrections you are making. And then save a copy for future reference.
https://my-cpe.com/uploads/add_material/1606139212_Handout3-Sample-Form-3115-for-depreciation-change.pdf

I'm going to follow-up on this post with a question. New client put a rental into service the end of 2021. House was set up for depreciation over 27.5 years, but should have been 39 year property (average of less than 7 days), and is still less than 7 days average for 2022. Does either a 3115 have to be filed here or an amended 2021 tax return? No way to just correct on 2022 return, correct? The difference in depreciation is only around $550 for 2021.

I'm going to follow-up on this post with a question. New client put a rental into service the end of 2021. House was set up for depreciation over 27.5 years, but should have been 39 year property (average of less than 7 days), and is still less than 7 days average for 2022. Does either a 3115 have to be filed here or an amended 2021 tax return? No way to just correct on 2022 return, correct? The difference in depreciation is only around $550 for 2021.

I believe I've read that if there is only one year's depreciation error an amended return can be filed. If more than one year's error, the IRS will consider that a change in accounting method requiring Form 3115. I could be mistaken.

No way to just correct on 2022 return, correct?

Correct.

A taxpayer who has used an impermissible method of depreciation for only the prior year has a choice to file Form 3115 for the current year or to amend the prior year. See section 6.01(1)(b) of Rev. Proc. 2022-14, 2022-7 I.R.B. 502, 515:

(b) Taxpayer has not adopted a method of accounting for the item of property. If a taxpayer does not satisfy section 6.01(1)(a)(i) of this revenue procedure for an item of depreciable or amortizable property because this item of property is placed in service by the taxpayer in the taxable year immediately preceding the year of change (“1-year depreciable property”), the taxpayer may change from the impermissible method of determining depreciation to the permissible method of determining depreciation for the 1-year depreciable property by filing a Form 3115 for this change, provided the § 481(a) adjustment reported on the Form 3115 includes the amount of any adjustment that is attributable to all property (including the 1-year depreciable property) subject to the Form 3115. Alternatively, the taxpayer may change from the impermissible method of determining depreciation to the permissible method of determining depreciation for a 1-year depreciable property by filing an amended federal income tax return, or an administrative adjustment request under § 6227 (AAR), as applicable, for the property’s placed-in-service year prior to the date the taxpayer files its federal income tax return for the taxable year succeeding the placed-in-service year.

Thanks all. That’s what I was afraid of. No easy way to fix for such a small amount.

No easy way to fix for such a small amount.

Oh, come on. Just flip it to 39-years effective 1/1/22 and move along.

I had one preparer in my area state that if they go from impermissible to permissible they don't even file the 3115.

Hmm, interesting!

This reminds me when I was working at a firm years ago, the owner would play around with depreciation on a lot of returns.
He would deduct depreciation in 1 year but not another, override the amount of depreciation, etc if he thought it would benefit the client more in that year or if he didn't want a future recapture when the asset will be sold. When I point out this was not allowed, he shrugged it off saying he's been doing it for decades and never had a problem.