Specially when it seems there was only the one Sun Stone. Although, one dialogue in Zeal seems to imply there was at least a second one the Gurus experimented on. That said, it all depends how they were housing it while using it. Maybe they had in the open to keep absorbing sunlight, thus charging up. Might not make much of a difference, but then, it depends just how much energy it was being used.
Should also be said that they also used the planet's energy in addition to the sun's, so perhaps between they shared the burden, so the Sun Stone wasn't draining as quickly. Although, dialogue in Zeal also says they stored the planet's energy in the Sun Stone, not just the sun's. That same one saying the Gurus tried to store the sun's energy in a Sun Stone. So maybe they were only using the planet's energy, then the Gurus tried to use the sun's as well, but then they found Lavos', and Zeal decided to make a full switch instead of using three energy sources.