I’m all in favour of long-term investing, but exactly how far-sighted can you afford to be? You may need binoculars to spot the ultimate rewards of investing in AIM-listed potash miner Sirius Minerals (LSE: SXX), as they remain such a distant prospect.

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One reason investors still get excited about SXX is that it’s looking to dig the world’s largest polyhalite mine not in Australia, Chile, Mexico or Mongolia, but right here in the UK, in the scenic North Yorkshire Moors. That’s also why investors have to be patient, as the company has had to negotiate the world’s toughest planning regulations, a process that’s now largely complete, mercifully.

The next stage is likely to prove just as arduous: securing at least £2bn worth of investment to fund the project and develop export facilities at Teeside. The fact that the mine will be dug beneath a much-loved national park will only add to the expense and effort. As does the collapse in commodity investment, although this can be overstated, as Sirius won’t be adding to the glut of metals on the market right now.

In a hole

Broker Liberum Capital has been positive about the stock’s prospects despite the lengthy timeframe, setting a target price of 38p. That suggested a 110% upside last November, when the stock traded at 18p. Today, with the Sirius share price slumping to 12.75p, the potential upside is nearly 200%.

One reason the share price has fallen is the wider stock market and commodity sell-off, which has dampened sentiment across the board. Since this says little about the prospects for this particular stock, some will see this as a good reason to buy into this potash-tastic prospect at a discount. On the other hand, it could make it harder for chief executive Chris Fraser to assemble that £2bn.

Going underground

There’s another reason the stock has fallen by a third lately. On 27 January Sirius announced a delay in its definitive feasibility study, which will now appear in March, two months later than planned. Management blamed the delay on the complex amount of information it must compile from various suppliers, consultants and engineering firms, essential for such a large-scale project. Investors were disappointed but they should have kept their heads. Given the long-term nature of this project, a delay of a couple of months is neither here nor there, unless it signals underlying problems.

Anybody who buys this stock must understand what they are dealing with: the mine has an expected life of more than 100 years. Piecing together the puzzle is laborious and you get no reward while you wait, because with no revenues, there’s no dividend either. There are signs of progress, such as the various polyhalite supply agreements Sirius is striking with major agri-business customers. This includes a seven-year deal to supply Chinese import and export group Huaken International with up to 500,000 tonnes a year, announced on Christmas Eve. But of course that means little until the mine becomes fully operational.

So yes, Sirius really could have a 200% upside. The question is: how long are you prepared to wait?

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Harvey Jones has no position in any shares mentioned. The Motley Fool UK has no position in any of the shares mentioned. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.