The motorway between Lisbon and the Algarve is a toll road. If you avoid the toll road then my guess is 4-6 hours depending on time of day. Lisbon is a nightmare traffic wise and you could spend 30 mins to an hour just getting out of the centre. Make sure you compare car hire rates as well: you may find it dearer to rent from Lisbon. Add the extra cost of fuel and it can't be worth it.
Pay a little extra to fly to Faro and be drinking a cold beer at your apartment an hour after landing. It's supposed to be a holiday, not an endurance test!

It's around 165 miles by road; my route planner says it will take about 2hrs 15 minutes driving. Most of the route is along a toll motorway. As the previous post says, the first part of the drive will be extricating yourself from Lisbon's suburbs, so maybe nearer three hours. The buses Lisbon - Portimao do it in four at the most.

If you are planning to need a car while in Porches, it would make sense to hire one at Lisbon airport.

thanks for the quick replies i will go to faro. the other family are going from lisbon and wonder if anybody has any idea of toll costs.thanks steve