Sandeep320
Śrī Mātre Namaḥ,
You can start or continue the 40 day practice during Navaratri but if you want to do some other sadhana for Navaratri as well please see if you can do both or else start or continue the 40 day practice after Navaratri.
It is always better to do less but do with full and sincere effort.
1 or 2 days break is fine but beyond that it is better to start again.
Śrī Gurubhyo Namaḥ.