Am i correct ?

[Scarredclown]

When i am in flight or in orbit, and i place a maneuver node saying i will need a burn of 25 seconds and i click on my tank and it says 290, it means that i have up to 29 secs burn at max throttle ?

[Pecan]

No. It means you have 290 units of fuel. How long that fuel will last depends entirely on which engines are burning it.

[Vanamonde]

Pecan is correct. The amount of fuel is of course proportional to the potential burn time, but different types and numbers of engines will burn that fuel for different actual spans.

[Kasuha]

Amount of fuel consumed also depends on atmospheric pressure. Most engines have different (higher) fuel consumption at higher atmospheric pressures. But in those situations you usually don't have time to place or follow maneuvers.

Also, even that 25 seconds burn time prediction is just an estimate and your burn will probably be a bit shorter because it does not account for fuel consumption (and subsequent acceleration increase).

[xtremespino]

As others have said, that's the amount of fuel. The time it takes to burn that fuel depends on the engine used.