In India, to reserve a berth in a train, one has to do it 1 to 3 months in advance of the actual journey or else reservations are not easily available. All this is due to high demand and low availability of trains. The time required varies by the season.
If one has to ask a help desk assistant about how long early one has to reserve the berth, how does one ask that? Is the following an idiomatic way?
How long early ( how long early should I make reservation?