i find that when they are really small it is sometimes hard to get them to eat as much as they should, but once they hit around 3-4" they eat more food and if you can get them on quarantined feeders, earthworms and some other live foods along with a lot of frozen foods and a little bit of pellets, they really start to take off.
My 1st senegal grew about 2.5" in less then a month, my current one has grown about .5" in a little over a month, but is starting to get to the point where he is showing some growth.
Once senegal's growth take off you can see it at an almost weekly basis.