Will 1/2 a syringe be enough for maintaining a previous injection along my smile lines?

It's possible, but it's difficult to answer effectively without pictures. I recommend following up with your injectors. I hope this helps. 

Hello and thanks for your question. Maintaining previous injections typically requires at least 1 syringe in order to provide a nice result. Please keep in mind that 1 syringe is a very small amount and would provide a modest result for a small area such as tear trough (would start with 1/2 syringe per side). Nasol folds and marionette lines require more than 1/2 syringe in most cases. All the best!!

Thank you for your question. 
Glad you are satisfied with your filler from one year ago. The syringes are not sold in 1/2 syringes.   Usually if one has volume loss in marionette lines, there is volume loss in the cheek area and  it is usually very easy to use one full syringe.  Often times there may be volume loss in the temple region or nasolabial folds.  Maintainance is great and using the full syringe should warrant nice results.
Please consult your surgeon injector or a local board certified facial plastic surgeon for more information and a comprehensive consultation