When they are around 8 months, sometimes 1 year. It all depends on breed and conditions. The Lionhead is a small rabbit.
First, you would have to know what type of Lionhead rabbit it is, Dwarf, Miniature, or Standard.

Can you tell me how old my lion head was when he stopped growing?
My lion head’s still growing but, she really isn’t putting on more weight. She just keeps developing muscle. Her last weigh-in was 1.8 pounds.
Dwarf Lionheads will stop growing at about 4 to 5 months old and should weigh about 2 1/2 pounds. Miniature Lionheads will stop growing at about 7 to 8 months old and should weigh about 4 to 5 pounds. Standard Lionheads will stop growing at about 8 to 10 months old and should weigh about 6 to 7 1/2 pounds.
My lionhead rabbit still isn’t finished growing at 2 years old. His weight is around 8-9 lbs now.
The Lionhead rabbit is a small animal, adults will be about 2-3kg on average, but there are exceptions of course, up to 5kg.
So, the Lionhead rabbit will stop growing at different ages depending on the breed, but will generally reach full size between 8 and 12 months old. Thanks for reading! I hope this article was helpful in answering your question.
FAQS
Q: My Lionhead rabbit is still growing, how big can they get?
A: The Lionhead rabbit will stop growing at different ages depending on the breed, but will generally reach a full size between 8 and 12 months old.
Q: My lion head rabbits are obese.
A: The Lionhead rabbit is a small animal, adults will be about 2-3kg on average, but there are exceptions of course, up to 5kg.
Q: How old do miniature lion heads have to be before they start having litter?
A: Around 7 months old.
Q: How old do Lionhead rabbits have to be before they’ve outgrown all their health problems?
A: Well, this depends on the problems. For example, a lion head with a respiratory infection will usually recover within a few weeks if treated properly. Some physical problems might take longer to develop while others can occur suddenly later in life. So, there is no definite answer to this question. Owners should be aware of the health problems their rabbits might have and seek prompt veterinary care if they notice any changes in their pet’s health.