Synthetic sleeping bag, how much insulation?

30F, plus or minus 5F depending in physiology.

What David says
certainly no lower.

Generally at most, a 4*C comfort rating is given for bags with cheap generic synthetic fillings of 200gsm. Primaloft will be better.

Do the maths. Work out your sleeping bags surface area, and account for insulation and 2 layers of fabric plus sundries. Compare the total to actual bags on the market of similar weight, and their temperature rating. (Montane use PL1)

My quick estimates compared with whats out there suggest that it won't be rated any warmer than 0*C. Most syn bags which are 0*C (32F) rated seem to come in at around a kg. Which suggests to me at least or more insulation than a single layer of 200g/m

Of course you may sleep warmer than the average , so it may be good for a lower temp.

Extrem do climashield apex too. It's loft will last much better than PL1, and it's significantly easier to handle/construct with.

Until recently PL's own website didn't recommend PL1 for sleeping bags – just their PL Synergy or PL Infinity (which are longer staple like Climashield – retains loft better).
info: http://www.shop-denali.com/types_of_synthetic_insulation.aspx