how long would it take

A very long time.

would it be worth my time

Almost certainly not.

Take the bronze ingot to iron ingot conversion for example. The mystic forge recipe calls for 1 iron ingot, 250 bronze ingots, 5 piles of shimmering dust and 2 philosopher's stones for an average return of 87 iron ingots. Each spirit shard buys you 10 stones so each one of your shards is good for 5 forgings. Do you really fancy the idea of clicking iron, bronze, dust and stones 60,000+ times? Even if you diversify your conversions (highly recommended unless you really want to destroy the economy and likely also your profits) and spread it out over weeks and months, you are still looking at hours and hours wasted at the mystic forge.

Unless you're willing to risk an account ban and set up an macro to do your forging for you while you're AFK, you are probably better off spending the time you would have spent in front of the forge in Istan or the like farming gold.