Larger projects will require regular inspections by the Architect, and may be required under the contract with the builder. On smaller projects you may be happy to inspect the work yourself; I won't try to sell you a service you don't want or want.
Experienced and Developer Clients :
I normally inspect the works at key stages or on request, unless agreed otherwise. This assumes that the client is sufficiently confident to deal with the technical issues which tend to arise during construction, and I can inspect more frequently if required.
Other Clients :
I would normally recommend that I inspect the works regularly, which normally means at least weekly, and more often at key stages.
Inspection by the Building Control Authority (Local Authority or Approved Inspector) :
The Building Control Authority's Inspector will visit site at key stages. However these inspections are limited to ensuring compliance with the Regulations, and do not cover quality or workmanship or finish, except where this affects compliance with the Regulations, and in any case the Inspector does not answer to the client. If the builder covers up work before inspection, they may have to dig it up/open it up for the Inspector !