In terms of a “standard infill” percentage, the default infill on 3D Hubs is 20%, but for most basic prints 15% infill usually works. However, for stronger parts (like mechanical, moving, or intricate parts), you can usually go up to 30-40% infill (so they don’t break)! You can print with whatever infill, but you should always confirm with the client first. Personally, I investigate the model and give them a suggested infill percentage, which they can then modify to their liking.
If you were wondering what infill is, it is basically just the inside of your part (It is also called Fill Density). It can be laid down in a specific pattern, but the “honeycomb” pattern is what I use the most. Different infill percentages will give your part different strengths. For example, an infill of 5% would give you a very weak part, however an infill of 50% would give you a very strong part. Unfortunately, the higher the infill percentage, the more filament you are going to use. That is why I never use a super high infill percentage for orders. However, if the customer requests a high infill percentage, you may want to consider charging them for the extra filament. You can see the attached picture for more details.
I hope this helps!