[ches]

When should I start and how often should I feel my tomato plants?

[New shoot]

If they are in pots, mine get a weekly feed.  While they are in the growth phase, they get a general liquid feed and I switch to tomato food once the first fruits set.

My outdoor blight resistant ones on the plot got a dose of Growmore on being planted and will get a handful or two more over the season of that, or pelleted chicken manure.