If California Law is anything like Georgia Law, then you can keep your Learner's permit for eternity, so long as you obey the rules required for the permit.
For instance, unless you have a true Driver's License, you must be in a car and riding with a licensed driver. You can't drive alone. That's a Learner Permit rule you will have to obey.
Of course, neither I nor this site are a good source to ask. I'd reccomemd viewing your state's website.
Though I am sure you can keep your Learner's Permit for eternity (at least in Georgia), you may have to get it renewed as you would a Driver's License.
Please note my answer is based off of Georgia Law, so it may not be accurate in your state.