A former client of mine was asked to prepare a 10-minute PowerPoint presentation to secure $100 million in new funding. "I could talk for HOURS on this -- how in the world do I do it in just 10 minutes?!" she asked me. "Well," I said, "every 60 seconds, just make $10,000,000 point...or a close approximation thereof."
So rather than ask how much you SHOULD personally spend on social media each day, ask, how much time per day CAN you ACTUALLY ALLOT to spending on social media -- especially in light of your current responsibilities, priorities and time commitments.
Then, gather a few employees (that are likely less than half your age!!) and, together, craft a strategy that will maximize your impact within the time allotted.
Think about it in terms of a corollary to Parkinson's Law, which states: Work expands so as to fill the time available for its completion – in your case, you want to insure that the work does NOT expand beyond the time available for its completion.
Without this front-end discipline...or a close approximation thereof, you'll likely spend far too much time for far too little return -- and who knows what else will fall through the cracks while you're busy tweeting away!