For me it depends a lot on the planes I have with me. I certainly love to fly in wind but that was not always the case. When I first started out much over 10mph was my limit, now that speed is just where the fun begins.

I'll think nothing of taking my 4*60 into a 20mph wind and hold it steady just a bit nose high into the wind. Sometimes I'll see how long I can hold (my best is 15 minutes, about 2/3rds of a tank) and others I'll land it that way, just get it stopped into the wind with the right amount of power and "fly" it down in one place.

At 20mph I'll consider not flying my Ultimate since that big rudder acts as a huge weathervane along with the wide interplane struts, but I will still fly every other plane I own.

As Ken said, when you are first starting out, only you can determine what speed is your limit. Skills have everything to do with this and some of the needed skills to handle wind can be practiced without wind like a slip. A crab does need some wind and is the planes natural stance in wind. For those that do not know what a crab is, watch a plane flying in the wind, both an RC and real one, (for the real one a small plane like a piper or cessna shows it well because they fly lower and are more susceptable to wind than any jet) preferably when the wind is a quartering wind, you will actaully see the plane flying sideways but going in a straight line in relation to the ground (ground track), that's a crab, the stroner the wind, the stronger the angle of crab. You can adjust the crab by turning more into the wind or less into the wind.

A slip is a lot harder and takes a bit of skill but can be practiced without wind. Be very careful at first since this will require the use of all the control surfaces and power plus the plane is now "cross-controlled" and could easily stall if allowed to slow too much. Do it up high and out in front of you, preferably flying away since it will let you see the effects better.

To do this drop a wing using ailerons just a bit, the plane is going to want to turn in that direction, counteract that turn with the opposite rudder but only enough to stop the turn, no more. Be careful, you will now lose some lift so the plane is going to want to decend so both power and elevator will now be needed to adjust the decent. With enough power and the right amount of elevator (IN THAT ORDER) you could level out. The goal is to keep the plane flying in a straight line in relation to the ground. It's not as easy as it sounds[X(]

Keep doing this until you get to the point where the plane just cannot slip anymore because you have run out of control authority which will let you know the limits of the plane and it becomes second nature. To land, hold the slip until a couple of inches above the ground and then release all the controls so the plane will straighten out and touch down just about a second later. This is a timing maneuver so knowing how your plane reacts to when you release the controls and get it straight again is why you must practice this up high

When you do this in the wind, the wind direction will dictate which way to drop a wing.

I'll repeat myself, the slip is an advanced maneuver but one that will reward you with loads of confindence once mastered, be very careful and practice it up high.

Oh yea, a slip is a great way to lose altitude quickly if you need to. Just do not add a lot of power or elevator