Pretty much right away- always included something I knew they would eat, and if it was something new had a back up.
Taste buds change so often as they grow that they may love something for 6 months the hate it for a year...
But, it's not a restaurant. If they don't want to eat, they don't eat. No extra mealtimes to make it up though.
ETA: DSS was a genuinely fussy eater, whereas DSD just wanted certain things.
Funnily enough, though, our strategy worked better with DSS. Maybe because he loved X so it was included in every meal so he always ate something.
Whereas DSD would just refuse until she got what she wanted- usually when she went to her mums.
Both are great eaters, with us, now. Not so much at their mums.