It'll probably be good for like 2 months and then they'll book stuff like shit again. But as for the actual split I see this lasting a long long long time if their churning out talents as quickly as they are now with NXT.
Based on their existing roster they have enough talent to make this brand split work. They have existing stars to work with like Cena, Orton, Rollins, Reigns, Wyatt, Lesnar, Ambrose and so on. They have a strong core of next tier down guys like Del Rio, Styles, Ziggler, Miz, and Cesaro. And they have talent who could be special if given the chance like Sami Zayn, Kevin Owens, Apollo Crews, Samoa Joe, and Finn Balor. So they have established talent and a lot of other talent that could help them maintain two strong brands. What they need to do is try and keep them separate and not do too much crossover stuff like what happened the last time round as time went on. I don't mind people breaking out on Smackdown then getting traded to RAW, to open up other opportunities for other wrestlers. But try and keep them separate and they have enough talent for two strong brands.
What should help them maintain a brand split long term is having NXT to help support them. It has already become a production line of talent for them, and as long as they continue to run it as they are, they are going to continue to develop talent that will ultimately make their way onto one of the main rosters. And with a brand split, that could help their future talent from NXT have an easier transition and help them gain opportunities which they may have had to wait longer for with a single roster.
As for a time frame, as long as they run it well, there is no reason why the brand split can't be a permanent thing because it gives them two main shows and a development brand.