is it a hardy outdoor geranium or the pot grown type ( pelagonium ). My outdoor hardies are still flowering but soon I will take the shears to them & cut them back to about 5" or so, they respond well every year to this.

If you mean a Pelargonium, then you can cut it back when you take it into a frost-free place in the autumn if you want to. I tend not to cut mine back now, as I find that they over-winter well anyway - I just remove any damaged leaves and any if they start to develop grey mould. I keep the plants on the dry side, too.

Geraniums/perlargoniums will keep going well into the autumn. Keep them picked over for marked leaves or shrivelled ones, dead headed, fed ,watered and listen out for frost warnings. If you need the space in the garden to plant bulbs, throw on compost or pot up 2 or 3 together and put in a sheltered place. You may need to huddle them and fleece them up later. If you have somewhere warm and protected they will keep blooming too. Last year I only protected mine when the severe frost warnings were issued in December. They stayed huddled under fleece or in a tiny plastic shelter until April.