The short answer is that a standard urban planter box usually maxes out at around 24 inches (60 cm) of internal depth. Most standard models you will find are closer to 12–18 inches (30–45 cm) deep, which is enough for annuals, herbs, lettuce, and many compact perennials. A 24-inch-deep box is generally considered the practical maximum for a “standard” urban planter because it offers enough soil volume for larger flowers, small shrubs, and some shallow-rooted edibles without becoming extremely heavy or expensive.
Larger commercial or custom planters can be 30–36 inches (75–90 cm) deep or more, but at that point they are usually classified as deep planters or custom installations rather than standard urban planter boxes. If you are comparing products, check whether the listed depth refers to the outside height or the actual soil depth. Drainage layers, false bottoms, and root barriers can reduce the usable soil depth by several inches.
For most urban gardens, 18–24 inches (45–60 cm) of soil depth is the sweet spot: it supports a wider range of plants while still being manageable on balconies, patios, and rooftops. Remember that deeper soil holds more water and weighs much more, so always confirm the load capacity of your site before choosing the maximum-depth option.