About 42,000 General Motors retirees have a big financial decision to make by Friday.

Salaried retirees who left GM between October 1997 and December 2011 are being offered the opportunity to take a lump sum pay out of their pension or to continue getting checks from an annuity which will now be run by the Prudential Companies.

Read more: GM forces tough retiree choices

Dean Thurman, a senior partner at InvestWise in Bloomfield Hills, said the offices have been swamped with retirees looking for guidance.

"I’ve been doing this for 22 years and typically take three or four appointments a day, and we’ve been taking 10 and 12 a day. It's been crazy,” said Thurman.

There is no simple formula to make the decision. Thurman said much depends on a retiree's other investments.

Kevin Hogle, who spent 31 years at GM, said for him the decision was easy.

"I looked at it and it probably took me five minutes to add the numbers up and divide and it just didn’t look like anything was going to work," he said. "Then I had my financial advisor look over it and it doesn’t work."

GM is not the only automaker offering the one-time lump sum option. In August, Ford will give 98,000 of its retirees the lump sum option. Best advice from financial experts? Always get a second opinion.