This case involves the question of whether an interspousal contract was formed between Deanna Stadnyk, the petitioner, and Ted Dash, the respondent, who lived together for $10$ years prior to separating.

On July $5,2007,$ Dash$$s lawyer received a proposal for settlement, which requested a response to the proposal within $10$ days. Dash was out of the province at the time, and some time passed before his lawyer was able to obtain instructions. Dash$$s lawyer replied on August $15,2007$ with an acceptance of the proposal, but Stadnyk$$s lawyer was on summer vacation, so the petitioner was not made aware of the correspondence of August $15$ until early September. In the intervening weeks, Stadnyk had reconsidered the offer made on July $5$ and instructed her lawyer to revoke it$.$

The position of Dash, the respondent, was that an agreement had been reached by virtue of his lawyer$$s letter sent August $15,$ accepting the offer proposed by Stadnyk; therefore, revocation was not an option available to the Stadnyk.

For Stadnyk$$s part, she contended that either Dash$$s acceptance was not made within the time prescribed for the offer or that, alternatively, the offer was revoked. Regardless, Stadnyk submitted that no settlement agreement was achieved.

Source: Stadnyk v$.$ Dash, $2007$ SKQB $443$

According to the text, for how long is an offer valid? Explain your answer.