$2-5 in cash is the standard range in the US, scaling up slightly at a high-end restaurant or hotel.
The tip is conventionally given when your car is returned to you, not when you drop it off — tipping at drop-off is less common and can read as an attempt to secure preferential treatment rather than a straightforward thank-you.
If a valet fee is already charged (common at hotels and some venues), a further tip is still customary on top of that fee — the fee covers the service itself, not a gratuity for the person who parked your car.
A typical sequence
Drop-off: hand over your keys, no payment expected yet.
Pickup: valet retrieves your car, you hand them $2-5 in cash as they open the door or hand over the keys.
Where this is much less common
Valet parking as a service exists mainly at hotels, upscale restaurants and some city venues, concentrated heavily in the US. In much of the rest of the world, where valet service itself is rarer outside luxury hotels, the US tipping norm described here does not reliably carry over, and a smaller or no tip is unlikely to be seen as unusual.
No cash on you? It happens. A quick, genuine thank-you is far better than nothing, though carrying a small amount of cash specifically for valet and similar quick-service tips is a habit worth building before a trip to the US.