A typical natural gas 50 gallon water heater will take about an hour to heat up.
The length of time required to replace a hot water heater will vary with the installation requirements. Installing an expansion tank or replacing saddle valves can add time to the installation.
Usually we spend an average of 2-3 hours replacing a hot water tank. It will take more time to sweat copper piping but makes for a better connection with less chances of issues down the road.
For many water heaters such as AO Smith, State, and Whirlpool you are better off letting the water heater sit directly on the floor even if it’s not perfectly level. The air intake for these water heaters are on the bottom of the tank which is protected by a pre-filter. If you raise up one of the edges to level the tank you are bypassing the pre-filter.