Insurance for a 390 Duke really depends on whether you're mainly riding it during the warm months or year-round. A lot of beginners overlook seasonal insurance, which is way cheaper - if you're not planning to ride the bike in winter anyway, you can save some serious cash that way. With your age and A2 license, you should end up somewhere in the lower to mid range, but it also matters whether you live in a city with high insurance zones or more out in the countryside.
It's tough to give you a realistic answer since prices vary quite a bit depending on the insurer and how they rate your personal risk. For me back then, it was also heavily tied to my no-claims discount on my car - if you've already got a good rating there, that often works in your favor. Check out multiple quotes (Check24, Verivox, and call insurers directly - online is often cheaper), try going for a higher deductible if that works for you, and specifically ask about seasonal discounts. Your annual mileage matters too - if you're not commuting every day, make sure you mention that.