Hi David,
We have a holiday home in the Limousin.... at the moment I am out for about 4 days every 3 weeks, but that is partially through a mad rush over renovations.....
I am self-employed and can take time off as and when I want and indeed work goes 'dead' for me in August. I can also do a fair amount of my work from home over the internet (I work 2/3 days at home anyway - so not difficult to transpose that to France).
In terms of travel times / costs the average time to Calais is about 7 hours for us, but then we live in SE London so we are only 1 hr from the chunnel. With car and autoroute tolls it is about £150 each way....I have the frequent traveller account with Le Shuttle and a 'widget' to use the toll roads (about E20 per year, then slight discount on the tolls).
However I also keep an 'old banger' in France. It costs me E500 per annum for a parking permit for the airport, and then we are an hour from the airport.... Limoges is served by 2 "low cost" airlines, so there is limited chance of the route being cancelled. Normally my Ryanair flights work out at about £65 each way - but if you book in advance you can get some genuine cheap deals - £30 one way, or even down to £15 (with taxes etc).
In Brittany your main route is Brittany Ferries (expensive and sometimes overnight) - although v pleasant.
Of course, we can't take the dog on the plane - so that rules that out for many trips.
Depending upon wheher you need to take a dog and / or the size of your family you may find that another region in France may actually be easier and cheaper if it is on a local low-cost air route for you..... Brittany may be physically closer, but properties more expensive...... for example in my region properties are dirt cheap.... what you save in property costs could go towards a car / parking / air fees. Check out your local routes and then explore the possibilities where they go.....
I have to admit that I think you would find the long drive a chore after a time if you were only able to go out for 2/3 days at a time. I certainly think that if this is all I could manage (sans chien) then I'd pretty soon tire of the journey. 5 days makes it a bit more worthwhile - so a 'night drive' and then recover....
All depends upon your work requirments I think: if you can extend your trips to a week (or just shy of it) then it may be worthwhile. If you are truly limited to 4 days you may find you don't use it very much. I also agree with the comments to the effect that there isn't much to be had by way of rentals - and what there is to be had will mean that you have to rent it out just when you want to use it ! You'll have to keep a higher maintenance spec when renting out and also, perhaps, set aside a room to put your personal stuff in when guests are there - you can't just arrive and put the kettle on !
If you can negotiate with work to extend your trips a bit (spedning a day or two working over the 'net) then it may be worthwhile. Alternatively if the low-cost airline option is a 'goer' then short trips may be ok. If not I fear that you may tire of those long journeys.....
RM.