it could take a long time, especially if the account you are merging from has a lot of transactions (eg it was used for a play). Always try to merge into the most used account, and if you need to afterwards, rename/renumber the account you combined things into.
Accounts must be of the same type to merge them. Eg, you can only merge an asset account into an asset account, an income account into an income account, etc. Since is possible to change some account types (eg you can change liability to income). If you wish to merge disparate account types, then change the account type first.