I was in Ushuaia in mid-april and had to deal with snow and black ice. Nothing too extreme, but the black ice was sketchy. A guy I was riding with was nearing hypothermia on our ride back north. Then again, he wasn't as prepared as he should have been. The small pass, just north of Ushuaia was the worst of it.
The road from Ushuaia north to the border into Chile is smooth well kept pavement. From border to border (going back into Argentina) is hard pack dirt. If its dry, cruising at 60 mph is no problem. It was wet on my way north and the hard packed dirt had a thin sheet of mud on top. Nothing to sloppy. It will just leave a cement like coat on your bike!
If it snows(its likely it will from early April on), road tires will not be fun but it should be do-able. The dirt section in the Chile side will be fine as long as it doesn't get to wet.
I would think you could do it in one, long day of riding. My suggestion would be to split it into two days of riding. Take a rest at a small town along the way. Can't really remember the name, but the approx. GPS coordinates taken from Google Earth are 54°30'36.24"S 67°11'51.52"W. Its half way between Ushuaia and Rio Grande. Here you will find a fantastic bakery (hopefully at the coordinates give above) that is very well known throughout Argentina. Damn nice place to warm up with coffee and baked goods. Even has an indoor "zoo" so to speak, with beavers, birds and etc. in glass enclosure. Go to the loo and you can watch the beaver play in its pool as you pee
. Yea, its kinda unique, but the baked goods are some of the best of the trip. A place not to be missed if you ask me.
If you do it in two days, stay at the Hotel Argentina(o?) in Rio Grande. Its at the south end of the "main" street(not the highway that kind of bypasses the town) going through downtown. Look for a old yellow building on the eastside of the street. This place was one of my favorite hotels of my 8 month adventure. Its owned and operated by Graciala(sp?), a wonderful woman that simply loves life. When we first pulled into the parking area, she was there waiting with a kiss on the each of our cheeks and offering us whiskey! The hotel is in a historical building and has more of a feeling of a hostel with character. Very laid back and you have free use of the kitchen and internet. There is also a nice living room with momentos of donated broken parts off of motorcycles, bicycles, shoes and more from all the adventurers passing through this lovely place. We found it heading south and loved it so much that we stayed there again on the way north and ended up staying for several days. Graciala to me, is known as Mama Argentina.
Hope this helps guys! Have a great ride. Any more question, shoot 'em my way.
Ryan