Generally it will only take a few days to get used to driving on the right. Many of the road signs are the same as at home and in our experience, people adapt very quickly. Roads in Portugal are well maintained and are far less congested than at home and there is an excellent motorway system.
Portuguese residents can not drive a car on foreign plates. If the car you are driving is left hand drive you can transfer to a Portuguese plate. A person wishing to change a right hand drive car can do so it they are new residents and already own their vehicle.
You can drive a UK registered car if you spend less than 183 days each year in Portugal.