At this time of year, off-season (Camino traffic wise), your best bet is after a Mass on a weekend. Aside from Catholic holy days, as mentioned by SYates above, there will likely be relatively few groups willing to pay to have the Botafumeiro used during the business week. It is logical really...
In my experience, peak Botafumeiro use can be expected during July and August, when more groups are willing to pay for the privilege. Also, there are several Catholic feast or holy days that call for using the Botafumeiro in July and August.
I would think that groups desiring to pay to ensure the Botafumeiro will be used, would do so on a weekend. but, this is NO GUARANTEE.
I hope this helps.