For planted tanks,
T8 should be replaced at 6 - 12 months.
T5 should be replaced at about 12 months.
Power Compact - for best results, should be replaced at 6 - 12 months.
LED - We have no idea, yet.
For heavily planted tanks using a fertilizer regime and pressurized CO2, aim for the low end, for the best plant growth. The quality of light matters more in high tech tanks.
For fish tanks with a couple of plants, don't worry about the lamp. Replace it when it's too dim, or blown.