How often can I make a sub-allocation?

As often as you need to make them. However, it is recommended that LIRs make use of a slow-start mechanism when making a sub-allocation for a downstream network operator. There are two main advantages to this: the LIR can ensure that the address space it sub-allocates is used efficiently; the LIR can also determine the ability of the downstream organisation to operate within the policies set by the RIPE community.