By watching their behaviour, it should be easy for you to tell if they are preening their feet or if they are biting with the intent of harming.
Does the budgie seem bothered and tries to break off the mate's advances on the foot? Do you notice bickering as one of the budgies tries to reach into the mate's foot?
There is not a one-size-fits-all answer as to the time it takes for budgies to start eating their fresh foods. Some can take longer than others.
The most important thing is to continue to offer them the veggies and trying to find out your budgies' preferences when it comes to how they want their fresh foods to be presented.
Some budgies like their veggies whole or cut into larger chunks and stuck in between the cage's bars, while others prefer them more finely chopped and placed on a bowl. To entice your budgies to try out the greens, you can sprinkle a few loose seeds onto the leafy greens.
A good starter veggies which budgies will usually go for is romaine lettuce ans for fruit, they aren't indifferent to a little slice of apple (no apple seeds as these are bad).
Since fruit is high in sugar content it's best to not offer it regularly, veggies can be offered on a daily basis.
For all of the detailed information, please check the stickies at the top of the Diet and Nutrition section.