The US-Mexico border is 1,900 miles or 3,100 km long. Let's be optimistic and say that, evidence to the contrary, he will complete his four year term without being impeached. How much of the wall will be built before the next president takes office and drops the whole thing?
And, if he manages to get it started (assuming that it's not just rhetoric) how soon would it be before the first sod is turned and the first filling laid?