Linseed can only extend the lifespan of wood if applied correctly and re-applied every couple of month. A once-off application will not do much to preserve the timber over time.
An answer to the second question is entirely dependent on the regularity of the treatment.

On a side note, the main enemy of timber is a wood-destroying fungus that thrives in moist conditions. Avoid constant overwatering, and give the wood a chance to dry out from time to time. Doing this will extend the planter lifespan considerably.