Around 1700AD
And would that be a small amount easy for any competent pirate captains to amass? Or would only the most successful captains get so much?
Assuming it's sterling silver, which is a big assumption, then 50 kg is about 134 troy pounds.
In the 17th century, 134 troy pounds of sterling silver is equivalent to about 8300 shillings, since 1 troy pound of sterling silver gives 62 shillings.
With 20 shillings to the pound, that's about 415 pounds.
415 pounds was quite a bit of money back then. For example, this bill of sale from 1680 listing the estate of Captain Valentine Bird, which includes a plantation, house, furniture, several slaves and indentured servants, dozens of livestock, etc. is valued at around 580 pounds.