I think it all depends on what portion of the lettuce you want to amplify. This paper targets CPTL2Article A Lettuce ( Lactuca sativa ) Homolog of Human Nogo-B Recepto...
Once you know which portions of the genome are specific for Lactuca Sativa you can design your own primers. For the other questions, you can check the other publications on Lactuca Sativa: they deal with bacteria more than the plant itself, but they say what kind of plant they used and how they were processed.
Far from an expert on this, but about the growth stage I guess it's about what you want to measure?
This article talks a bit about the number of leaves : Article Selection of reference genes for diurnal and developmental t...
These ones are more about duration i guess :
Article Elucidating the genetic basis of antioxidant status in lettu...
Article Transcriptomic analysis reveals the roles of gibberellin-reg...
Then again the amount of material would be depending on the devices you have at disposition, maybe you can ask someone in another lab experienced with the qPCR instruments you can use and even though they might not work on the same subject as you, their experience can reassure you when you work on your own protocol.