In general, the IRS likes to see a profit for 3 of the previous 5 years.  That said, there is no actual limit on the number of years your business can claim a loss, rather all facts and circumstances would be considered by the IRS before disallowing the losses and converting the business to a hobby.  The below list of items to consider are from this IRS link: IRS Hobby or Business?

Generally, an activity qualifies as a business if it is carried on with the reasonable expectation of earning a profit.   In order to make this determination, taxpayers should consider the following factors:

• Does the time and effort put into the activity indicate an intention to make a profit?
• Does the taxpayer depend on income from the activity?
• If there are losses, are they due to circumstances beyond the taxpayer’s control or did they occur in the start-up phase of the business?
• Has the taxpayer changed methods of operation to improve profitability?
• Does the taxpayer or his/her advisors have the knowledge needed to carry on the activity as a successful business?
• Has the taxpayer made a profit in similar activities in the past?
• Does the activity make a profit in some years?
• Can the taxpayer expect to make a profit in the future from the appreciation of assets used in the activity?