After 20+ years of not logging 121 time, I'd like to rebuild a logbook and keep up to date for post retirement opportunities.

Any ideas of what is industry standard/acceptable for a retiring 121 pilot that hasn't kept a log book up to date?

I do have about the last 5-6 years accessible from a company/union database.

Thanks in advance!

I think rebuilding a 20-year logbook will take a considerable amount of time and effort. If you have flight records from the outfit(s) where you were employed and are able to put all of them all together in an orderly fashion, that should suffice. It is unlikely that any interviewer will make a big deal of it.

that’s more than adequate

I did the same. Had to convert my paper log books into an electronic one. I chose LogTen Pro which obviously a lot of people use. Unfortunately it’s a yearly subscription, so not a cheap option. So far I’m happy with it. The print outs worked well for an interview logbook review. You will have to sit down a considerable amount of time to get it done. Took me weeks to insert all my 15K+ hours… But it’s a nice review/ memory for yourself of a lot of flights you might have forgotten about.

Depends on the situation.
I would go with company records and training events.
Probably by year.

Type of airplane / hours flown / year
So you’ll have 20 lines.

Look at FAR 61.51 and what is required for an official logbook. The one line entries are not sufficient to meet the requirement as an official logbook, but you may not need it depending on the employer and what they require. Note that you are not required to log all of the time that you have flown. You just need to log the time required for a particular rating, certification, or other as required by FAR.

So, I would hope that the last 5-6 years would be sufficient for any non-airline job. Especially if you can show that you worked for airlines for the last 20+ years.

FAR 61.51 is official for faa purposes. Once you have an ATP there are very few things you need to log, none of which is of any relevance after 24 months

for hiring/insurance purposes a basic overview of airline experience is more than adequate

nobody will care whether a retired 121 pilot had 10,000 hours or 20,000

Company kept records are compliant obviously and for interview purposes your records don’t need to be.
A future 135 employer would be much more interested in recently failed check events.

Prospective employees of 135/121/125/129 can’t hide from PRIA any longer. FAA has gone electronic and DMS, IACRA, MEDEX send results directly to the airman’s FAA file.

Just curious how most people are logging their flight time. When I started flying 30+ years ago it was a simple book, paper and pen. I don't fly professionally (all part 91), just fly one aircraft (RV7) and use fltplan.com (a Garmin subsidiary) for everything. I get all the charts and its free. They have a elog book which is so user UN-friendly. I know most use Foreflight. What electronic flight time logging programs do most use second to Foreflight?