The longer the better. In my experience the dogs most likely to need special behavioural help going forward are the ones who only got a short period of stay-at-home support when settling in as pups. This isn't invariable of course!
It really is a big project, I'd consider using most of your annual leave.
If you're planning to use dog walkers or daycare going forward worth involving them right from the beginning.
Last summer one of my doggy clients died, aged 14. I'd first met her when she came to her new family aged eight weeks, at the beginning of the school summer holidays. I popped in for her several times that holidays when the family had to go out. Then I was visiting three times a day two days a week when term started, reducing to twice, then once, and lengthening the walks. I continued to walk her on the days when neither mum nor dad were working from home right up to last July, when I had the heartbreaking call that she had suddenly collapsed and died during a weekend walk with her whole family.
We love being involved in the puppy stage, but it's important to make sure the pup doesn't get confused about who their owner is. Another client only had a week with one of the couple, followed by a week with the other half, and then only saw me during the weekdays.
She absolutely refused to obey one of the two owners but would do anything for me. Not a satisfactory situation at all. She was a large and powerful breed. The family eventually moved away, but I was not surprised to hear from mutual friends that when the couple had a baby they re-homed the dog.