With a Deposit Boost, it's most common that the loan would be repaid in the event the house was sold. There may be an early repayment charge if the loan is still within the initial offer period. If there is an expectation that the property may be sold after a specific period (eg: 3 years, 5 years) then the loan can be set up to match that time. It can always be extended if the property doesn't end up being sold.
There is usually the potential that the loan can be ported to the booster's next property if they aren't ready to pay it back yet.
For more Booster details, check out our guide.