We have to replace our oven which is 220v. When they came to install the new 220v oven, they said that the current oven was shaing a line with the 110v gas cooktop and that we have to have separate lines. What does this involve and how much would it cost and how many hours would it take for an electrician to do this?

I need to understand your current configuration.

Are you saying the 120 Volt receptacle is being sourced by one leg of the oven? If so, this is a code violation. Ovens require a higher amperage than you can safely use on a 15 or 20 amp receptacle.

The 240 volts for the oven and the 120 volts for the cooktop need to be on separate circuits.

You also need to know if the new oven requires a four conductor plug.

If your current installation is a three wire plug/receptacle, you may have to install a complete new 240 volt circuit for the oven, depending on the installation instructions for the oven.

Regardless, the 120 volt receptacle cannot be part of the 240 volt circuit.