It depended on your profession and some other factors, like how good you were, whether you had a record, in some cases your ethnic origins, party membership, etc. In general, for a good Russian engineer or accountant it was not a problem to switch jobs after in most cases served the first 2-3 years after college at a narrow choice of in demand offerings that government tried to fill in semi-compulsory way, often away from home (like major industrial projects or teaching in remote countryside). On the other hand, if you were a Jewish space engineer, switching jobs could be a big problem. In general, people who switched jobs every couple of years were not regarded high, if it was not an obvious career rise.