"If Scotland is to have a real choice - when the terms of Brexit are known but before it is too late to choose our own course - then that choice must be offered between the autumn of next year, 2018, and the spring of 2019," Ms Sturgeon, who heads Scotland's pro-independence devolved government, told reporters.
The prospect of an independence vote in Scotland that could rip apart the United Kingdom just months before an EU exit would add a tumultuous twist to Brexit with uncertain consequences for the world's fifth-largest economy.
British Prime Minister Theresa May is poised to launch the two-year process of taking the country out of the EU, something which was opposed by most Scots in last year's Brexit vote.
Ms Sturgeon has called for Scotland to be allowed to strike its own deal with the EU but on Monday she said her efforts had hit a "brick wall" in London.
The results of the June 23 Brexit referendum called the future of the UK into question because England and Wales voted to leave the EU but Scotland and Northern Ireland voted to stay.
Pound Sterling rose after Ms Sturgeon said the earliest date for a new Scottish independence referendum was in the autumn of next year. British government bond prices fell.
Scots rejected independence by 55-45 per cent in a referendum in September 2014, though the vote energised Scottish politics and support for Ms Sturgeon's Scottish National Party (SNP) has surged since then.