As Huddleston and Pullum (2002) note, you can (optionally) treat such expressions as singular. They give such examples (§3.4, p. 354) as:

• That ten days we spent in Florida was fantastic.
• That two pounds of sugar you bought isn’t going to be enough.

Note that this only works with "measure phrases," and that the quantity has to be preceded by a (typically small) cardinal number or by the word "few."

So either is acceptable in this case. That said, given the length of the duration in question, treating the phrase as plural might be preferred; using the plural may also be considered more formal.

As Huddleston and Pullum (2002) note, you can (optionally) treat such expressions as singular. They give such examples (§3.4, p. 354) as:

• That ten days we spent in Florida was fantastic.
• That two pounds of sugar you bought isn’t going to be enough.

Note that this only works with "measure phrases," and that the noun has to be preceded by a (typically small) cardinal number or by the word "few."

So either is acceptable in this case. That said, given the length of the duration in question, treating the phrase as plural might be preferred; using the plural may also be considered more formal.