In addition to setting loan lengths for All Item, by Item Type, and on individual items, you can also set loan/rental lengths based on Membership Type. You can even specify loan length for particular items based on membership type while setting defaults for others.
1. Set the loan length on your membership types in: Settings -> Users -> Membership Type and edit each Membership Type
2. Go to Settings -> Inventory -> Set Item Default Values
3. Select "All Items" or the specific Item Type you'd like to set to use the Membership Type default loan length (it will also apply to all subtypes of that type).
4. When editing the Type, change the option to "Use Membership Length" from Enter Length
5. Save the changes. Now, when checking items of that type out, the default return date will be based on the value set in the membership type rather than your overall default or length based on item type.
*Note: you cannot have different default loan lengths for different types of items by membership type. This would apparently cause our developers heads to explode and we love our developers and do not want to see them harmed.