Hello marmalade1,
I flew my RV-4 for the first time this season. You had asked how long I can fly mine (we're both running OS .70 Surpass II's). I have the timer set on my Futaba 7CAP for 12:30 (I always forget to set the thing - but today I remembered to switch it on for each flight). I flew mine for a little over 12:30 one flight and I still had about a quarter tank left on my 12 ounce tank - so I would say 10 to 12 minute flights should not be a problem. Remember - I'm spinning a 12 x 9 - if you're spinning a 13 x 6 you might be a little more fuel efficient. I hardly ever move the throttle any higher than 3/4 - that thing tears right around at 1/2 to 2/3 throttle so that's about how heavy I usually am on the hammer with this airplane.
How are you for ground clearance with that 13 x 6? The tips of my 12 x 9 "turn a little green" in the grass at my field but I've never had any taxiing incidents or nosed it over. I'm considering switching to a 13 x 6 - but am hesitant to try that since I'm mowing the grass a little with my 12" prop as it is.
I would refer to this airplane as a "high speed cruiser" with the .70 Surpass II - it's heavy but it can move and it does fare well in the wind. It climbs well and does nice stall (hammerhead) turns - it also has a good aileron roll rate as well. Loops are not a problem either but you need to carry good speed into them and all the way over the top to make them look pretty (weight issues with the airframe balanced for the .70 Surpass II). My RV4 also flies well inverted.
Here's a shot from my field today - weird winds from periods of calm to 10 - 15 mph from the north, east, south and west (more than a couple of times I took off in one direction and landed the opposite). The shot shows the Top Flite Contender I'm breaking in - my RV4 is in the background.
Happy Flying!