18% here - I'm thinking that's likely to be on the high end of most ranges. This topic is front of mind, since we just paid 1st half school taxes today (all but 5 of our properties have school taxes due Oct & Mar).
We are in a low cost and high tax area (Southern Tier NY). So tax as % of gross rent should be relatively high vs. other regions. However, functionally those high taxes are passed on to renters -- rent as a % of property value is relatively high, even though absolute rents are not expensive.
All property taxes in our area include 4 categories: county/town, city/village (only if property is in an incorporated area), school, and sewer. Water bills can lien the property but aren't technically taxes, so not included. Property taxes total 17.9% of gross rents for the easy-to-calculate subset (38 deeds, 71 doors total). For in-city properties (all but 7 deeds), this includes trash pick up.
When we were in CT, the mil rates varied drastically from town to town (or city). However, I noticed that out of pocket taxes paid tended to be similar on a per-door basis. That is, a median $450,000 home in a "nice" town might pay about $4,000/year in taxes, and a median $120,000 home in a "low income" city might pay about $4,000/year in taxes.