Are you growing from seed?
Spearmint is part of the mint family which is hardy to invasive. When the conditions are good, I can cut 2 plus foot mint stems down to a couple of inches above a leaf nodule and get 2 foot stems again within about 4 to 6 weeks. In good conditions with pruning, I can get leaf buds to put forth reasonable sized leaves within a couple of days; of course, pruning encourages fast growth/re-growth.
Good conditions include a compost rich, acidic soil that is reasonably moist but not sopping wet. On an absolute scale, lightly filtered sunlight (shaded if California type) and air flow as they are prone to powdery mildew. They do better with some ambient (air) moisture but not too much and/or dependent upon air flow and soil moisture as they are prone to powdery mildew. They seem to grow best between 60 and 75 degrees. Ground level as opposed to overhead sprinkler watering is advised.
Mint nnaturallyseeded in the ground with minimal attention and under only somewhat decent conditions, tends to go from seedling visibility (1"-2") to a scraggly mint plant with leaves of about 6"-12" within 6 to 8 weeks. Within 3 months, it can be a plant that you know will survive some harvesting and into the future.
Spearmint plants, at least the types that have been available in the garden centers over the past 10-15 years, are a lower growing, more bushy, and less hardy plant. Still, they share much of the same characteristics. The various other mint cultivates like pineapple, chocolate, and more are not near as hardy. On the other hand, the plant we call lemon balm is self mounding, a little slower to get established, quicker to seed than mint, but just about as hardy as mint; it does like a little more sun than mint and handles drier conditions with more visual appeal than mint.