In situ reflection high energy electron diffraction (RHEED) has been used to study the time evolution during self-assembled molecular beam epitaxy (MBE) growth of InAs quantum dots on GaAs. Using a special data acquisition technique, two characteristic time constants are determined very precisely: the time t(c) up to the first appearance of InAs dots and the time t(f) it takes to complete the 2D-3D transition of all islands. Surprisingly, we find that t(c) increases with temperature which disagrees with a thermally activated process. In contrast to this, t(f) behaves Arrhenius-like and an activation energy of E-f similar or equal to 0.39 eV is determined. Furthermore, the sum t(c) + t(f) does not depend significantly on temperature and corresponds to an InAs coverage of similar or equal to 2.0 monolayers. A second focus of this paper is the study of dissolution of InAs dots after interruption of the As flux. From the experiments, an activation energy of 3.2 eV for desorption of In located on top of the wetting layer is determined, whereas direct desorption from the wetting layer corresponds to an activation energy of 3.4 eV.