I have a unit cube, and operating in the continuum limit (i.e. not on a lattice), I sequentially place spheres of some radius $r$ inside the cube until a filled volume "jamming limit" $\theta_{spheres}$ is achieved s.t. no further spheres can be placed inside of the cube. Is there an accepted range of values for $\theta_{spheres}$ in the literature?

For circular discs in the unit square, we can find a value of $\theta_{circles} \approx 0.547 +/- 0.003$ in the literature$^{1,2}$. See this ( Packing density of randomly deposited circles on a plane ) MathOverflow question and:

Hinrichsen, E.L., Feder, J., Jøssang, T. Geometry of random sequential adsorption. Journal of Statistical Physics 44(5-6), pp. 793-827 (1986).

Cadilhe, A., Araujo, N.A.M., Privman, V. Random sequential adsorption: from continuum to lattice and pre-patterned substrates. J. Phys. Cond. Mat. 19, 065124 (2007).

However, I'm having trouble finding such a range for the 3D case with spheres? Also, is there an accepted value for packing spheres of radius $r$ into a larger unit sphere?

In terms of running simulations, I seem to be hitting a "soft" wall around a value of $\theta_{spheres} \approx 0.335$ for packing $r = 0.02$ radius spheres in a unit cube. I say "soft" because further spheres can still be added with (what seems like) an exponentially growing number of attempts at placement.