For NAND flash-based storage systems, managing garbage collection (GC) efficiently is a critical requirement to achieve both high performance and long lifetimes. In this paper, we propose a just-in-time GC technique, called JIT-GC, which invokes background GC operations only when necessary depending on future write demands. JIT-GC was motivated by our measurement study, which strongly suggested that deciding when to invoke background GC operations is a key parameter for efficient GC. By accurately estimating the amount of future SSD writes, JIT-GC can choose the best time to invoke a background GC operation. JIT-GC reserves necessary free space in advance so that high write performance can be achieved while it extends the SSD lifetime by preventing premature block erasures. Our evaluations on real SSDs show that JIT-GC can achieve both high performance and long lifetimes, thus overcoming the shortcomings of existing background GC invocation heuristics.