That will all depend on what profile you are using. There are many t-slot profiles small to large and they also come in different weights for example 15 series, 15 series light, and 15 series ultra-light. You can see deflection formulas here https://parco-inc.com/t-slot-deflection-calculations/ but it may be easier to figure out how much weight you need to carry and then call one of the distributors or manufacturers. There is a list on this site.