Post when your customers are off the tools
There is no magic minute that works for every trade, but local customers tend to browse before work, at lunch, and after dinner. Start with one of those windows and watch which posts get saves, replies, and profile visits.
For most trades, two dependable posts a week will beat a burst of seven posts followed by a quiet month. The useful routine is the one you can keep on a busy week.
Build the post at the end of the job
The easiest time to post is while the job is still fresh in your head. Take the finished photo, write the one-sentence brief, and prepare the post before you leave. Scheduling can come later; capturing the detail now is the part that matters.
Use your local time, not a generic rule
If most of your work is in one area, your customers share a rhythm. Test a lunchtime post against an evening one for a month, then keep the window that brings the most useful replies rather than chasing likes.