Sweet pea seedlings

I nipped the tops off my Sweet Pea seedlings a few weeks ago but the new branches have become rather long and lanky.
Should I cut them back, and if so how short should they be ?

 

We would, nip them as you would the main shoot.

They say starting them off in Autumn or Winter gives a stronger earlier plant, but always find they have grown too fast and lanky plus a big tangle of roots.
We just sow in Spring so we do not have to keep stopping them which in a way must be better for them ?

 

For many years I started sweet peas off in the greenhouse in the autumn and I found they grew lanky and got tangled and in many years took a while to get going when planted out. This year, because last autumn sowing was struggling, I sowed another set in early spring, Root Trainers in greenhouse. These grew a lot better (more light?) and grew strongly when planted out. So nexy early spring is penciled in for sowing sweet pea seed.

 

Thank you both, I have always sown them in the spring but thought I'd try autumn this year. I think I'll be doing it in the spring from now on

 

I think lankiness is due to the greenhouse being too warm, the old gardener I started with started his in a cold frame, and these developed into sturdy but compact plants, having been stopped at least twice. They were spaced at least a pot width apart.

 

Most plants that try to grow under glass in winter end up leggy in my experience.
The advantage of a cold frame is total ventilation when ever the temperature is above freezing.
Less problems with fungus as well.