First thing I'd ask; are they in a position to receive at least 5 hours of sunlight in the day? Most inexpensive such lights don't store energy very well and don't last through the night unless rcieving sufficient sunlight...
Clanad's spot on there. Someone gave us a host of cheapo lights that were unwanted, and to get decent, long-lasting light, we found we had to leave them in direct sunlight for hours. Leave them switched off all day, to harness the power, and then turn them on when it gets dark. Providing sufficient energy's been stored up, then your lights should keep going for a good while.