Usually, a 7-day tour can cover the highlights of Nepal with the cities of Kathmandu, Pokhara, and Lumbini. A mountain trekking tour will take 7 days, too. is available. Mount Everest Base Camp trek requires two weeks. If Nepal is just a transit stop for a day or two on the way to Bhutan then a quick scoot around Kathmandu and neighboring Patan will be good.
If you have a holiday of two weeks in Nepal, you will have enough time to cover the best of the Kathmandu valley, do a short trek in Langtang, and do a quick safari in Chitwan National Park. For three weeks in Nepal, you can add a longer trek such as Everest Base Camp or Annapurna Sanctuary. For a month in Nepal, you can discover most of the historic sites and towns all over Nepal including one long trek (Everest Three Passes, Annapurna Circuit, Manaslu Circuit, and Kanchenjunga are all possibilities) or two shorter treks.