Chafer Grub Treatment
As soon as your nematodes have found their host (Chafer grubs in your case), death of the host usually occurs by 48hrs. Environmentalfactors (temperature, moisture, pest location, etc) will play apart in how effective the nematodes are, but if you can keep conditions favourable for at least 2 weeks then they should work well. These conditions should be stated on the application instructions, and will vary slightly depending on what specie of nematode you are using.
I would personally say you could apply your grass seed the following day after your nematode treatment as its going to take at least a few days to even begin to germinate, but certainly after a week or two.
Pest Control Expert