October through February is the best window. Daytime temperatures sit around 26–30°C, mornings are cool, and the granite of the temple compound is bearable underfoot. The light is also at its best in these months — the low winter sun comes through the temple's stone windows and lights the ceiling mural directly for an hour either side of midday.
March to June is hot and dry, with afternoon highs over 38°C. The temple stones radiate heat well into the evening. Come at first light if you must visit in summer. The monsoon (July to September) is brief here — Anantapur district is one of the driest in India outside Rajasthan — and rarely disrupts a day trip, though clouds dim the mural.
The temple's annual chariot festival (Brahmotsavam), held in February, draws crowds of pilgrims. If you want to see the place at its liveliest, time your visit then; if you want it empty, avoid the festival week.