Most currency options only go out to 12 Months and beyond that straight-line interpolation of annual vols works very well. As for Forwards there will be some distortions, notably here options expiring after potentially volatile days e.g. elections will have markedly higher vols than for those expiring just before, but again these events are quite rare and visible to all. GBP Put 12.8 1.53 1.31 Priced at market vols, the combination of options is worth 6,740,000. This sort of difference could drastically alter the assessed profitability of a trader, especially if these trades were systematically done. Yet too few market prices will leave us with the dangers outlined earlier.
Many compromises are in use.
We are still left with the issue of interpolation of vols across the 2-dimensional surface as we only have a few points dotted about. We can now just straight-line interpolate across periods and we have our complete surface as required.