A normal ROD or a descent without a specified rate is considered to be 1000' /min. Do less and in most cases your next call will be to increase your ROD.
A will have a look if I can find this in a certain document and let you know. But at the place where I work 1000' is considered standard.
If you are unable due to technical problems to comply, but this goes for any rate, please advise ATC asap so they can take into account in the situation.