WIll this box hold a 35 pound light fixture?
If the screws are course thread and driven solidly into the ceiling joist the box should hold your luminaire without a problem.

You could test it by hanging 35 pounds of weight from the fixture strap for a few days.

Standard ceiling boxes are manufactured to hold up to 50 pounds if they are solidly installed.

Here is the pertinent National Electrical Code Article:


  314.27(A) (2) Ceiling Outlets. At every outlet used exclusively for lighting, the box shall be designed or installed so that a luminaire or lampholder may be attached. Boxes shall be required to support a luminaire weighing a minimum of 23 kg (50 lb). A luminaire that weighs more than 23 kg (50 lb) shall be supported independently of the outlet box, unless the outlet box is listed for not less than the weight to be supported. The interior of the box shall be marked by the manufacturer to indicate the maximum weight the box shall be permitted to support.


That is a "pancake" box so your fixture should have a canopy that adds more volume for the wire connections since there is not very much room in that box.