What is a "reasonable" amount of time for a toddler/preschooler to take to eat dinner? How long is too long?

30 minutes. Patience is key. Allow them to try different food items and try a variety of foods as well. Taste buds change quickly at this age so if they don't like it now retry after 6 months.
More info. So in answer to your question, there really is no right amount of time for eating a meal. Toddlers will eat when they need to and you can't change that. This is one of the "losing battles", so struggling with your toddler can only make things worse. Only have healthy food & drinks available in your home so they don't fill up and learn to crave junk food. Don't be a short order cook.
Toddler eating. Most toddlers only need 1 or 2 meals a day, so they dally at meal(s) they don't need. The trick is not to try to get them to eat when they aren't hungry & stop struggling. Avoid between meal snacks & drinks. Mealtimes should be low key & pleasant-- no "clean up your plate" or "eat your broccoli" families should eat together and parents should set a good example. Tension at meals ruins appetites!