The shower in your RV will use around the same amount of water as your regular shower in your home depending on the shower head and it's gallon per minute.
A typical shower whether in an RV or in your home uses around 15 to 20 gallons of water for an 8 minute shower so the longer you shower the more water you'll use and the less time you shower the less amount of water you use.
In an RV if you're using the water holding tanks for water then you'll need to limit your shower to around 5 minutes to conserve water but if you're hooked up to a public water supply at an RV park then you can take as long as shower as you want as long as your water stays hot enough in the RV water heater.
RV water heaters are also small and can usually only give you around 5 minutes to shower with hot water before it needs to heat back up again.