How often should the meter be read and by whom?

The distributor is the entity that installs the meter, maintains it and takes the readings. The distributor must attempt to take readings: 
at least once a year for customers with consumptions up to 500 SCM/year;
at least twice a year for customers with consumptions from 500 SCM/year to 1,500 SCM/year;
at least 3 times a year, for customers with consumptions from 1,500 SCM/year to 5,000 SCM/year;
at least once a month for customers with consumptions from 5,000 SCM/year.
For example, if a customer consumes 1,000 SCM of gas per year and the distributor visited for a reading on 1 February, the customer can expect a further visit in the following autumn, as the distributor must try to include at least 80% of the reference period for each reading (1 year, 6 months, 4 months depending on the customer's consumption). 

In the case of new supply activations to customers consuming less than 5,000 SCM/year, the first reading attempt must be made within 6 months from the day of service activation. 
The distributor may make plans to take readings that are better than those required by the Authority, informing the Authority and all suppliers on its network. In addition, it must publish the calendar of the organised reading visits on its website at least by municipal postcode.

We speak of an attempt and not of a reading that has certainly been taken, because if the meter is located inside individual houses, the customer is not always present to let in the meter reader sent by the distributor. Even if the meter is not in the house, it may be necessary for a person to open a door or gate to let the meter reader into the room or place where the meter is located.
If the customer is absent, the distributor leaves a notice stating that they were unsuccessful and inviting the customer to contact their gas supplier to communicate the self-reading.
The distributor must also collect a self-reading that the absent customer may have left on the doorstep (so-called "post-it").
If the distributor has failed to take a reading and has not found or received the customer's self-reading either, it will make an estimate using all available previous readings or self-readings and the standard withdrawal profile (i.e. the consumption trend curve based on season and customer type).

The actual reading, the self-reading collected by the distributor or the estimate calculated by the distributor is sent to the supplier, which will use it for billing in accordance with the regulation (if the customer is in the protection service) or the contract (if the customer is in the free market).