They do not know the length of time of incubation. 
ðeɪ du nɑt noʊ ðʌ lɛŋkθ ʌv taɪm ʌv ɪŋkjubeɪʃʌn. 
 Indian's voice is very clear and has a very monotone tone in this recording. The sound is slightly close-sounding, giving the impression of being in the same room with the speaker.