(Adds details, analyst quote) By Shida Chayesteh COPENHAGEN, Dec 19 (Reuters) - As Danish group Novo Nordisk awaits final approvals for its new long-actingdiabetes drug Tresiba, a Reuters poll found a wide range offorecasts for sales of a drug meant to challenge its biggestrivals. Tresiba's annual peak sales were seen in an 8-100 billionDanish crown ($1.4-$17.7 billion) range in the poll of 18analysts. Forecasts by analysts from international banks saw thebiggest span, while those by Danish peers were more uniform. The highest estimate of 100 billion crowns in annual peaksales was given by Swedbank analyst Johan Unnerus. "It is probably towards the high end but that is my guess,"Unnerus said. The most optimistic analysts see Tresiba sales peakingalready in 2017, while those with the lowest estimates do notforecast this to happen before 2025. Novo Nordisk, the world's biggest insulin producer, hasestimated that Tresiba, which is the brand name for the activeingredient degludec, could become a blockbuster drug. A drug reaches blockbuster status when it generates morethan $1 billion in annual sales. Tresiba is central to Novo's aim of ending rival Sanofi's dominance of the long-acting insulin market. Itexpects to launch the new product in several European marketsearly next year, subject to final approval. Most analysts expect the U.S. Food and DrugAdministration(FDA) will approve the drug, as it has alreadybeen approved by regulators in Japan. Last month, an advisory panel to the FDA voted to recommendapproval of Tresiba, despite signals of possible cardiovascularrisk. The FDA rarely goes against its advisory panel'srecommendations when making its final decision. One of the reasons for the big span in the estimates is thatTresiba's sales are likely to continue to rise until anotherproduct takes over, said DNB analyst Rune Majlund Dahl. "Normally in the industry, we're talking about 10 yearsgenerally, which is when you hit a 'patent wall', but that isnot the case with diabetes products because we historically haveseen that sales just continue," Dahl said. "You would probably still have seen rising sales of Levemirif it were not for Tresiba," he added, referring to Novo's otherinsulin drug. "It gets a little difficult to estimate more than five yearsinto the future," said Jacob Thrane, analyst at S&P Capital IQ. "Especially when you can see there are a few things in thepipeline at the moment both at Lilly and GlaxoSmithKline, and we must not neglect that Sanofi is goodto keep up its Lantus (sales)," he added.($1 = 5.6458 Danish crowns) (Writing by Victoria Klesty; editing by Stephen Nisbet andLouise Heavens)