A ten percent increase is reasonable, twenty five percent is probably pushing things a little. Keep in mind that if you raise prices too much too fast, you risk scaring off your most serious collectors. Instead of continuing to buy, they stick with what they already own, get giddy about how much it's gone up in value and search out new and more affordable artists in hopes of duplicating the good fortune that they experienced with you. Remember that your loyal collector base is responsible for whatever level of success you enjoy, so keep them coming back for more. You might even think about offering your longest standing supporters special price breaks or buying opportunities from time to time.
Some artists advocate raising prices a fixed amount per year, say ten to twenty percent.
Arguments in favor of this practice center around offsetting increased costs of living or receiving annual raises like other working people. These are not necessarily good reasons. For one thing, employees whose job performance does not improve over time do not get regular raises. In fact, they may even lose their jobs. If you don't visibly improve your work, your exhibition record, and your sales volume to the point where the art community takes notice, raising your prices becomes arbitrary and unjustifiable.