[en] The relation between free volume size and the lifetime of ortho-Ps trapped in that volume is usually described in the framework of simplified model, proposed by Tao and Eldrup et al. In that model the positronium trap has spherical shape and the potential is rectangular one with infinite depth. In spite of its simplicity in most cases the model gives proper values of o-Ps lifetime and its zero-point energy in the trap. It can be also easily extended toward nonspherical and large-size free volumes. In that last case the population of particle state lying above the zero-point energy has to be taken into account. (author)