What's the optimal way to space out rewards in order to maximise motivation? I'm working on an educational product, and we're thinking of introducing some gamified aspects to motivate students to use the product. One of these will be the concept of 'levels' - when a student does enough lessons in a particular topic, their 'level' in that topic will increase. 

Is there any kind of research on how to best space these 'levels' out in order to maximise motivation? If students can progress through levels too quickly then leveling up will be devalued, but if it takes too long to get through levels then they may be less motivated because the end won't be in sight. How long should we aim for students to take to get through a level? 1 week? 1 month? 1 year? 

Any advice on how to approach this problem would be greatly appreciated.
To add gaming element to education, you have two options:


Look at currently successful examples of products that gamify learning
Check most successful mobile games (short engagement times)


Here are a couple examples of language learning products that gamify learning (Duolingo and ChineseSkill) They use:


Very short lessons which produce instant sense of gratification (3-5 minutes each)
Diverse word sets, so each lesson feels fresh
Score and timing concepts (for example Chinese Skill app lets you peek at a word meaning, but subtracts points from the final score for doing so)
Competition against "general public" - apps tell you how you are doing compared to other people
Use of pretty graphics and friendly imagery


For example chinese skill app starts with just a couple lessons per module, each focusing on a few words. Later on the app expands and starts to add more lessons. 

Duolingo has similar approach, although they have a lot more reward systems - you can win points by doing good during lessons. 

To specifically address your timing question, I can bring up personal experience - I tried both products, and hit a slump when the number of lessons per module exceeded 5. There was less motivation to progress. I quit duolingo completely once I hit a module with 13 courses. To avoid this, I've seen some apps break large word sets into stuff like "Clothing 1", "Clothing 2", etc. 

ChineseSkill app


Duolingo App: