I think he's getting close, it could go either way though so I will make the case for both.
First: I'm operating under 1 assumption, the fact that to my knowledge Slater has NOT been charged with anything therefore there's no heat on him yet.
Case to keep him: it's his first offense with drugs, the he serves his suspension then returns, during the suspension, the investigation into the assault concludes the person who accused him was looking for a payoff to get rich and Slater is proven innocent. Vince has no scandal to worry about and Slater returns to goes back to jobbing on Superstars and in dark matches, collects his pay check...life continues
Case to fire him: it's his his first offense with drugs, during the suspension official charges are filed stemming from the assault, WWE gets attacked for their involvement, Vince decides the scandal isn't worth it and Slater is wished the best in his...crap it's trade marked...
I think he's gonna get fired, WWE does seasonal cleaning and Fall Cleaning time is coming up right around survivor series. I can easily see him coming back and being destroyed by Mark Henry then wished the best in his...(trademarked 2 word phrase we all love). Slater is a jobber with no charisma and he had nothing outside of Nexus...even jobbers need to be able to sell the beating they take to elevate the talent above them.
Final Verdict: Slater's gone by the Royal Rumble at the latest