Often tumors will spring up and grow rapidly, but plateau for LONG periods of time.
I have a girl who's got two fair-sized tumors, that don't inhibit her quality of life or her health in any way. Although lumps are upsetting to find they are not always a death sentence! She has had one for about 10 months, and the other for about 3. She is my oldest girl currently and outside of the lumpies, is the healthiest of my "old" rats.
If you can afford a spay and removal, that's great. But if you can't, you may not need to worry for some time yet.