As BeeStar has said , the first 10 bought within the first hour . Then after 4 hours , another 10 bought . Finally , at 8 hours the final 10 bought equaling up to 30 Helms .
The GE limit is 4 hours for everything, as far as I'm aware. Each type of items varies and has its own limits though. Just like No Rage Quiting said above me ^.
If you follow this link to the RS Wiki, you can find a chart that shows you the time limit and the amounts you can buy for every class of item you can think of:
Within that 4hours you are allowed to buy/sell through GE unlimited number of times. However the most number of items you are allowed to purchase is the limit
Example:
Limit 20/4h
Buy 20
Sell 10 (you can purchase another 10)
Buy 10 (limit reached)