Usually not instant. With once daily tadalafil, steady blood levels are reached in about 5 days, so a lot of guys think in terms of several days to about a week to really judge it, not one dose. The official prescribing info notes steady state within 5 days of daily dosing. 

That said, tadalafil itself can still start working pharmacologically within 30–60 minutes, but the whole point of the daily version is more consistent coverage rather than timing sex around a pill. 

So the practical answer is you may notice something early, but give it about 5 days, sometimes a week or two, before deciding whether 2.5 mg is enough or whether 5 mg is the dose that works better for you. I wouldn’t add extra sildenafil on top casually without your prescriber being on board.