We've heard over and over that setting timetables would be a big mistake, that it would show a lack of resolve, embolden our enemies, etc. In most other situations, the inability to set a timetable shows the exact opposite: either that those presenting the plan aren't committed to it, or that the problem is not, in fact, understood. To say that we can't set a timetable is to say that we don't in fact have a reliable plan, that if our enemies knew even that much about our plans, they could disrupt them. It's an admission of weakness, not of strength.
The hilarious part of this (I laugh myself to sleep thinking about it every night) is that those who argue most strenuously against timetables invoke them regularly. How often have we heard that the next six months are the key? Don't such statements invoke a deadline, a timetable if you will, for dramatic improvement?