You can contribute to Law Research indefinitely, however, it becomes increasingly expensive to do so. Each Law requires different Resources to grow, and you’ll notice that the price to research increases as you increase the research level. Eventually, you will also see that the Resource cost converts to Crowns. While the Crown cost is more stable, it will still increase, to a set maximum.
Your contributions reset each day, so you can manage your economy to maximize your effectiveness. Additionally, you sometimes get a multiplier applied to the Research Points you generate, which makes your donations even more effective! The chance you will get a multiplier increases as you upgrade Parliament, giving you incentive to upgrade your Parliament quickly.