The R44 has between 30 and 45 mins of battery life after the red ALT light illuminates. The POH is 'land as soon as practicable' so you do not need to dive into the nearest field. In most of the UK, this will mean an urgent divert to the nearest airfield, with a stop watch running to ensure you keep WELL WITHIN the timings.
If this is not possible then locate a suitable secure landing site)next to big house, etc) as an engineer will appear at some point within 24hrs with a new or replacement alternator.
(I've had this problem - it was a loose alternator pillar. The red light winked on and off for a while then stayed on. Divert ensued. This could be a bigger issue over water or in remote areas. Make the radio call as soon as it happens).
For the R22 the figures are "similiar". I did once take a R22 in for maintenance with the alternator out as the second pilot on board. The J/T was 25 mins.
In all cases, minimise electrical usage. Take out the second radio/ GPS/ lights if not dark/ etc to keep amp usage down and watch the Ammeter.
Imagine trying to fly with Govenor, Horn, and both tachs out.
For this point try the Robinson Safety Course. It can be done but care needed to avoid overspeed (and stall). Engine sound is the clue but better not to be in the situation.