There's no "customary" percentage.
A house might be in great condition and need only minimal fixing-up. Let's say $3,500 for paint, minor repairs, etc. On a $100,000 house, that would be 3%. On a $300,000 house, that would be 1%.
On the other hand, a house might need moderate fixing-up. Let's say some work in the kitchen and bathrooms. A bit of landscaping. And painting and carpets. So, maybe $20,000. Now, I wouldn't put that much into a $100,000 property (see Gerard's comments). But that'd be 10% on a $200,000 property, but only 2.5% on a $500,000 property.
So: No customary percentage.
What you want to do is fix/correct any potential deal-killers. Don't fall for the rationale that "I don't know if I want to spend all that money on _______ when the buyers might not like what I selected." It your carpeting is ratty, replace it. If you've got 20 year old wallpaper, strip the wallpaper and paint the walls in an off-white or light beige. People are looking for properties in move-in condition. And they tend to greatly overestimate the cost of repairs.
After you've fixed the deal-killers, focus on a couple of items/areas to really make them "pop." So the entire property is fully acceptable--clean, workable, move-in condition--with a couple of areas that will really "wow" the buyers.
A home stager can give you some very good guidance on what to tackle.
Hope that helps.