If we knew how to reliably double our money, we'd all be doing it.
You could put it all in savings and at current interest rates you'd double it in about 50-60 years.
You could put it in the stock market, and you could double it in under 10 years, or lose the lot, or have it grow much more slowly.
In practice, nobody can project out that far because it's dependent on many and complex factors. Also a factor is how any growth would be taxed, or whether you can actually gain from the tax system (e.g. putting it in a pension vehicle).
But generally, the sooner you want the money, the higher the risk you'd have to take.
Personally, if I had 30K but needed 60K, I'd protect the capital and find another way to get the other 30K.