How long should I wait before I get my Restylane injections "topped up"?

While it is possible that Restylane can only last 3 months in the lips, it usually lasts up to 6 months if the appropriate amount is being used and the injector is good. There is also the slight possibility that the injector is using an inferior product or a 'black market' product from overseas that could be fake.
In order to make sure of this, insist that you get the box tops from the product in order to send it in for 'Restylane Rewards'. If they won't or can't give it to you, you probably are getting fake or foreign product. This means you ought to go elsewhere.
The other possibility is that they are trying to do too much with too little. If a patient comes to me and wants to get too much done and refuses to do the appropriate amount, I decline to do the procedure. Make sure they aren't using too little as the pain in this area to get it done means they ought to do the right amount so you don't have to do it every three months.

The bottom line is that the answer to your question depends on personal preference. If you are not happy with your results after 3 months, then you should return for more.
Before returning to the same physician, we'd ask whether or not the product only lasts three months every time as this seems to short a duration to us.
Research shows and we have found that using dermal fillers can actually aid in the development of new collagen and this is one compelling reason to use the products regularly. When the correct amount of product is injected into the smile lines the produce should last 4-6 months and closer to 6 if it is used consistently.

Good Day Maley,
You can always add to filler already in place if you wish for more correction.  You need not wait for the product to dissolve.  You may want to try Perlane (a thicker, sometimes longer lasting form of Restylane)  in the future.  Be well and enjoy your Vegi-mite.
Dr. P

As long as you have waited well past the initial, expected "post-treatment swelling" period (maybe even up to a month or so), it is fine to add additional volume of the hyaluronic acid fillers if you feel you would like more volume or line correction.

Restylane and Juvederm do wear off faster in the areas with more movement (smile lines faster than under eye tear troughs, for example), so needing to "top off" filler in these areas sooner does make more sense.

There is scientific evidence that over time, the placement of these fillers in the dermis actually does stimulate your own body to build more collagen, so doing a few more injections may also help your body create its own permanent collagen rebuilding which can help your long term outcome even more.

See a board-certified, careful, and artistic dermatologist or plastic surgeon for your treatments.

Injectable fillers like Restylane are essentially a camouflage for the wrinkle, however there is a lot of evidence to suggest it generates natural collagen in your skin and may help support your skin preventing further damage (wrinkling).  There is no doubt that Restylane is a very successful product when used in the smile lines.  More than half of my patients have results that are obvious for greater than 12 months and many have lasted 2 or more years.  Because of the protective effect that I believe Restylane imparts on the skin, I think it is to your advantage to not allow it to fully dissolve prior to a touch-up procedure.  In addition, you will avoid the roller coaster swings by doing smaller amounts more often.