I've been growing an avocado from a seed for something like half a year now, maybe more, and I don't know when to plant it in the garden.
It's about a meter high (approximately 3'2"). When should I plant it?
Now is a good time but you need to harden off the plant before planting it in the ground. Start taking it outdoors for 3 or 4 hours a day for a few days and then lengthen the time outdoors so as to get the tree used to weather. Also, it appears that your tree is in need of fertilization as evidenced by the leaves starting to turn purple and the leaves showing yellow-green between green veins. I would fertilize with both micro and macro nutrients.