Napoleon is an admired figure nowadays, but he is also still controversial. With an individual who had such a great effect on history there will almost always be great debate and disagreement surrounding the basic question-was he good or bad overall? Just as he's admired nowadays by many historians, he was also admired by many back then, if not least for his military skills.

Whilst the countries he fought against, conquered and occupied would generally have a negative view of Napoleon he was also praised by many of his contemporaries for his revolutionary politics and ability to manage and organise France into a great power dominating Western Europe in the chaos of the revolutionary period, as well as his military skill. The story of his rise from a basically middle class background in Corsica to becoming the Emperor of the French also made him a symbol of French revolutionary principles and won him admirers. In the New World in particular Napoleon's defence of and later extension of the revolution's principles to its neighbours was seen as a positive campaign to free Europe from the old order, similar to the US' own War of Independence decades earlier.

In Europe itself however Napoleon was generally viewed with contempt-especially amongst the elites and powerful of the old order powers-Britain, Russia, Austria etc, who fought him, as well as Spain who fought a brutal guerrilla war against his forces. For the ordinary people caught up in his campaigns Napoleon would unlikely have been praised as a great strategist and leader. There were exceptions of course-Napoleon's emancipation of the Jews, protestants in Catholic countries and Catholics in protestant countries won him support amongst those groups, whilst also earning enemies, the Pope and Russian Orthodox Church for example. His support for a Polish state in the form of the Duchy of Warsaw made him a very popular figure amongst Poles, who formed Polish legions that fought with French forces during the revolutionary and Napoleonic periods.

Amongst most Europeans however by the time of his defeat he would have been a disliked figure. His death came as a shock to both Europe and the New World and kickstarted an ongoing historiographical debate surrounding the man. Unlike some historical figures there has never been a synthesis or consensus on Napoleon, some historians writing today maintain he was a brilliant leader, (see the recently released and highly recommended 'Napoleon the Great' by Andrew Roberts), others that he was responsible for mass destruction and the setting back of European economic development, (see 'Napoleon: A Biography' by Frank McLynn which is generally a negative assessment of Napoleon). Dutch historian Pieter Geyl compared Napoleon to Hitler, but most historians would disagree with that assessment, even if they thought Napoleon a largely bad figure. If you want a now, slightly out of date, but nonetheless interesting study of the development of historiography surrounding Napoleon try Geyl's 1946 'Napoleon For and Against.'

The answer to your question therefore is that there isn't a set date where Napoleon became an admired figure. For some historians and nations he's been admired ever since he took power, for others he remains a bad, if not evil, figure. The debate will likely continue for quite some time.