If the Su-35 deal is true, its a probably a politicised decision for what Beijing government perceives on threat environment. This was the same when Australia purchased 24 F/A-18Fs as interim fighters even though their first F-35s are arriving in 2018, and had its own share of controversy.
If this is the same for China, then 24 Su-35s were bought as 'interim air-superiority fighters' until the J-20 development is completed and first squadron of J-20s are fully operational. This might also mean J-20 program is going to take longer than first thought since imported Su-35s will take years to be all handed over to the PLAAF. This is all pure speculation, assuming the deal is real with confirmation from the Chinese side.
Rumour has it that the first regiment of J-20s will enter service no later than 2017. It is even possible that the J-20 and the Su-35 (assuming deal goes through) will achieve combat operation capability at around the same time.