I would prune it just before new growth starts in early spring. That way it will recover quickly and won't look a mess for long.

I would decide on a range for it to fill, and snip off anything that goes beyond it, at any time of year. Ivy is a real menace (on old buildings) and will thank you for pruning it by growing more!! I find a good implement to prise off the tendrils from good brickwork is a wallpaper scraper! You may also need to trim back the mature flowering parts (these have different shaped leaves and grow out, away from the support).

I have a couple of gardens with ivy all along the fence - I cut it back flat to the fence in about May, and then again in late August, September, which keeps it under control.

If you like ivy, and I do, then you'll want to preserve it for its value to wildlife. Therefore, prune/tidy up in early spring. That way you keep the berry for the birds, and allow it to make good growth for more flower late in the year. Worthy