not sure what you mean.
depending on how many larvae you have (or big/small species)7.5 liters may or may not be enough. you will have to see for yourself.
Late L3 larvae also eat less then early L3, so that's a factor too.
I give my D.granti and D.tityus 0.5 gallon every month in early L3; 0.5 gallon every two months for late L3. and they are medium to small size rhino beetles.
I lightly press on the substrate when I put them into the container (don't press when you have larvae inside the container!!!) Not-pressed substrate is only about 50% of the Firmly pressed substrate too...
If possible, I would suggest to prepare extra substrate so you can add it in whenever you need to.