Citrus leafminer is usually active from summer through fall and into winter. It normally can only attack new leaves, so avoid pruning your citrus trees, because pruning can cause new flushes of growth. Likewise, because lemons put on several flushes of growth, they are more likely to be attacked by the pest.
You may not need to spray anything to control citrus leafminer. In established trees older than 4 years, no yield reduction or damage to the tree (other than aesthetic) results from their feeding. So if you can tolerate looking at the trails made in the leaves by the leafminer larvae, you don't need to spray. On younger trees, you may need to spray something like horticultural oil until new growth hardens off.