I started playing with small salads after about a month - doc had added veg to my diet after ten days as my protein intake was already more than adequate at that time. I generally use chopped spinach rather than lettuce owing to its somewhat better nutritional profile; for some reason. lettuce seems to be one of those things that some have a hard time tolerating for quite a while, though I had no problem with it.
I usually made them as part of a normal protein based meal rather than an add on. I cut back the meat of a normal meal from a nominal 3 oz to 2 oz, then added a few grams each of chopped spinach, grape tomato, avo, snap pea, scallion, cheese, bell pepper, etc. (whatever we had around...)