For the LTV-FHE scheme, after how many additions should modulus switching be used?
First of all, I want to clarify that there is no relinearization involved in ciphertext multiplication. 

Second, as discussed in the BGV paper, its not the ratio of noise to modulus size that matters but the absolute magnitude of noise is important and we should bring down the noise magnitude after each multiplication. 

And finally comparing homomorphic addition to multiplication, the noise growth in multiplication is exponential whereas the noise growth in addition is atmost a single bit. Therefore we can perform multiple additions but not multiplications. Theoretically, you can compute a bound for maximum number of additions or multiplication before the noise wraps around the modulus, rendering the decryption unsuccessful.

I hope this helps you.