The Russian Grand Prix is set to be a one-stop race, the only question is when to make the switch from ultra-soft to super-soft (or vice-a-versa if you starting outside the top ten).
Relatively low temperatures combined with a smooth track surface mean Sochi has always been easy on tyres and ideally Pirelli would have an 'uber-soft' compound for tracks like this one. In 2014, Nico Rosberg wrecked a set of tyres by locking up into the first corner, but still managed to complete the rest of the race on a single set of softs in order to fight his way back to second place.
In previous years, teams used to balance setup between graining on the front tyres and overheating on the rears, but the new Pirelli tyres are less susceptible to graining meaning the focus is now on keeping the rears in the right operating window. Both the ultra-soft and super-soft are low working range tyres, meaning the compounds are designed for cooler conditions like the cars experience in Sochi. The soft is a high working range tyre and is very, very unlikely to make an appearance at any point on Sunday.
In terms of physical wear, Pirelli's tyres should be able to go the full race distance, so the focus will be on how much performance is eking away from the rear tyres lap after lap. Valtteri Bottas completed the most laps on a single set of ultra-soft tyres during Friday practice, with 32 (24 of which were on heavy fuel and the rest on an earlier low-fuel run). By the end of the long run, Bottas was going faster than he was at the start, suggesting the lowering of the fuel load was counteracting any degradation from the tyres.
Considering the race is 53 laps, drivers should be able to run the ultra-soft tyre for a large part of the race before making a pit stop -- offering a huge amount of flexibility over when to switch to the super-softs. The key will be to avoid getting stuck in traffic as overtaking is not easy at the Sochi Autodrom and a lack of degradation means the fresh tyres won't offer the instant pace advantage (so often used to gain an 'undercut' on a rival) that they do at other tracks.
Throw in the high chance of a Safety Car and race strategy at Sochi is not quite as simple as its low-degradation/one-stop reputation makes it seem. But with a clear track and no outside influences, Pirelli's data suggests a stop on lap 26 (the midway point of the race) is optimal.