There are many things to consider. First of all we know that measuring work performance can be, more often than not, a flawed process. Also 120 days is about right when you consider an employee needs a verbal warning and a written warning with a performance improvement plan to take effect.
30 days sounds like an insufficient amoubt of time to me. Moreover, 120 does sound like a lot but it's just 4 months when you think of it.
I said it once and I'll say it again, the best defense against poor work performance is having good hiring practices. More often than not however employers see HR as a needless cost. Some of my friends are business owners and come to me looking for advice on what to do with their underperforming employees. I immediately ask them about their hiring procedures and more often than not I get "oh someone I know referred me this person" or they didn't check references or they went with their instinct cause they liked the way he/she interviewed. SMH